Geometry instructions for decompression of three-dimensional graphics data

ABSTRACT

Three-dimensional compressed geometry is decompressed with a unit having an input FIFO receiving compressed data bits and outputting to an input block state machine and an input block, whose outputs are coupled to a barrel shifter unit. Input block output also is input to Huffman tables that output to the state machine. The state machine output also is coupled to a data path controller whose output is coupled to a tag decoder, and to a normal processor receiving output from the barrel shifter unit. The decompressor unit also includes a position/color processor that receives output from the barrel shifter unit. Outputs from the normal processor and position/color processor are multiplexed to a format converter. For instructions in the data stream that generate output to the format converter, the decompression unit generates a tag sent to the tag decoder in parallel with bits for normals that are sent to the format converter. The decompressed stream of triangle data may then be passed to a traditional rendering pipeline, where it can be processed in full floating point accuracy, and thereafter displayed or otherwise used.

CONTINUATION DATA

This application is a divisional of U.S. Pat. Ser. No. 08/511,326("Method and Apparatus for Decompression of Compressed GeometricThree-Dimensional Graphics Data" by Michael F. Deering and Aaron S.Wynn) which was filed on Aug. 4, 1995, now U.S. Pat. No. 5,842,004 andis hereby incorporated by reference as though fully and completely setforth herein.

FIELD OF THE INVENTION

The present invention relates generally to decompressing graphics data,and more particularly to methods and apparatuses that decompresscompressed three-dimensional geometry data.

BACKGROUND OF THE INVENTION

Modern three-dimensional computer graphics use geometry extensively todescribe three-dimensional objects, using a variety of graphicalrepresentation techniques. Computer graphics find wide use inapplications ranging from computer assisted design ("CAD") programs tovirtual reality video games. Complex smooth surfaces of objects can besuccinctly represented by high level abstractions such as trimmednon-uniform rational splines ("NURBs"), and often detailed surfacegeometry can be rendered using texture maps. But adding more realismrequires raw geometry, usually in the form of triangles. Position,color, and normal components of these triangles are typicallyrepresented as floating point numbers, and describing an isolatedtriangle can require upwards of 100 bytes of storage space.

Understandably, substantial space is necessary for three-dimensionalcomputer graphics objects to be stored, e.g., on a computer hard disk orcompact disk read-only memory ("CD-ROM"). Similarly, considerable timein necessary for such objects to be transmitted, e.g., over a network,or from disk to main memory.

Geometry compression is a general space-time trade-off, and offersadvantages at every level of a memory/interconnect hierarchy. A similarsystems problem exists for storage and transmission of two-dimensionalpixel images. A variety of lossy and lossless compression anddecompression techniques have been developed for two-dimensional pixelimages, with resultant decrease in storage space and transmission time.Unfortunately, the prior art does not include compression/decompressiontechniques appropriate for three-dimensional geometry, beyond polygonreduction techniques. However, the Ph.D. thesis entitled Compressing theX Graphics Protocol by John Danskin, Princeton University, 1994describes compression for two-dimensional geometry.

Suitable compression can greatly increase the amount of geometry thatcan be cached, or stored, in the fast main memory of a computer system.In distributed networked applications, compression can help make sharedvirtual reality ("VR") display environments feasible, by greatlyreducing transmission time.

Most major machine computer aided design ("MCAD") software packages, andmany animation modeling packages use constructive solid geometry ("CSG")and free-form NURBS to construct and represent geometry. Using suchtechniques, regions of smooth surfaces are represented to a high levelwith resulting trimmed polynomial surfaces. For hardware rendering,these surfaces typically are pre-tessellated in triangles using softwarebefore transmission to rendering hardware. Such softwarepre-tessellation is done even on hardware that supports some form ofhardware NURBS rendering.

However, many advantages associated with NURBS geometric representationare for tasks other than real-time rendering. These non-rendering tasksinclude representation for machining, interchange, and physical analysissuch as simulation of turbulence flow. Accurately representing trimmingcurves for NURBS is very data intensive, and as a compression technique,trimmed NURBS can not be much more compact than pre-tessellatedtriangles, at least at typical rendering tessellation densities.Finally, not all objects are compactly represented by NURBS. Althoughmany mechanical objects such as automobile hoods and jet turbine bladeshave large, smooth areas where NURBS representations can beadvantageous, many objects do not have such areas and do not lendthemselves to such representation. Thus, while NURBS will have manyapplications in modelling objects, compressed triangles will be far morecompact for many classes of application objects.

Photo-realistic batch rendering has long made extensive use of texturemap techniques to compactly represent fine geometric detail. Suchtechniques can include color texture maps, normal bump maps, anddisplacement maps. Texture mapping works quite well for large objects inthe far background, e.g., clouds in the sky, buildings in the distance.At closer distances, textures work best for three-dimensional objectsthat are mostly flat, e.g., billboards, paintings, carpets, marblewalls, and the like. More recently, rendering hardware has begun tosupport texture mapping, and real-time rendering engines can also applythese techniques.

However, texture mapping results in a noticeable loss of quality fornearby objects that are not flat. One partial solution is the"signboard", in which a textured polygon always swivels to face theobserver. But when viewed in stereo, especially head-tracked VR stereo,nearby textures are plainly perceived as flat. In these instances, evena lower detail but fully three-dimensional polygonal representation of anearby object would be much more realistic.

Polyhedral representation of geometry has long been supported in thefield of three-dimensional raster computer graphics. In suchrepresentation, arbitrary geometry is expressed and specified typicallyby a list of vertices, edges, and faces. As noted by J. Foley, et al. inComputer Graphics: Principles and Practice, 2nd ed., Addison-Wesley,1990, such representations as winged-edge data structures were designedas much to support editing of the geometry as display. Vestiges of theserepresentations survive today as interchange formats, e.g., WavefrontOBJ. While theoretically compact, some compaction is sacrificed forreadability by using ASCII data representation in interchange files.Unfortunately, few if any of these formats can be directly passed asdrawing instructions to rendering hardware.

Another historical vestige in such formats is the support of N-sidedpolygons, a general primitive form that early rendering hardware couldaccept. However, present day faster rendering hardware mandates that allpolygon geometry be reduced to triangles before being submitted tohardware. Polygons with more than three sides cannot in general beguaranteed to be either planar or convex. If quadrilaterals are acceptedas rendering primitives, it is to be accepted that they will bearbitrarily split into a pair of triangles before rendering.

Modern graphics languages typically specify binary formats for therepresentation of collections of three-dimensional triangles, usually asarrays of vertex data structures. Thus, PHIGS PLUS, PEX, XGL, andproposed extensions to OpenGL are of this format form, and will definethe storage space taken by executable geometry.

It is known in the art to isolate or chain triangles in "zigzag" or"star" strips. For example, Iris-GL, XGL, and PEX 5.2 define a form ofgeneralized triangle strip that can switch from a zigzag to star-likevertex chaining on a vertex-by-vertex basis, but at the expense of anextra header word per vertex in XGL and PEX. A restart code allowsmultiple disconnected strips of triangles to be specified within onearray of vertices.

In these languages, all vertex components (positions, colors, normals)may be specified by 32-bit single precision IEEE floating point numbers,or 64-bit double precision numbers. The XGL, IrisGL, and OpenGL formatsalso provide some 32-bit integer support. The IrisGL and OpenGL formatssupport vertex position component inputs as 16-bit integers, and normalsand colors can be any of these as well as 8-bit components. In practice,positions, colors, and normals can be quantized to significantly fewerthan 32 bits (single precision IEEE floating point) with little loss invisual quality. Such bit-shaving may be utilized in commercialthree-dimensional graphics hardware, providing there is appropriatenumerical analysis support.

However compressed, geometric data including three-dimensional geometrydata must be decompressed to be useful. For example, applicant's patentapplication Ser. No. 08/511,294 filed Aug. 4, 1995, entitled METHOD ANDAPPARATUS FOR GEOMETRIC COMPRESSION OF THREE-DIMENSIONAL GRAPHICS DATA,assigned to the assignee herein, discloses such compression.

Thus, there is a need for method and apparatus for decompressingthree-dimensional geometry that has been compressed. Preferably,decompression is such that the output data may be passed to renderinghardware directly as drawing instructions. Finally, decompression ofthree-dimensional geometry should be implementable using hardware,software, or a combination thereof.

The present invention discloses such decompression.

SUMMARY OF THE PRESENT INVENTION

For decompression according to the present invention, three-dimensionalgeometry is first represented as a generalized triangle mesh, whichallows each instance of a vertex in a linear stream to specify anaverage of between 1/3 triangle and 2 triangles. Individual positions,colors, and normals are quantized, with a variable length compressionbeing applied to individual positions, colors, and normals. Quantizedvalues are delta-compression encoded between neighbors to provide vertextraversal orders, and mesh buffer references are created. Histograms ofdelta-positions, delta-normals and delta-colors are created, after whichvariable length Huffman tag codes, as well as delta-positions,delta-normals and delta-colors are created. The compressed output binarystream includes the output Huffman table initializations, ordered vertextraversals, output tags, and the delta-positions, delta-normals, anddelta-colors.

Decompression of such compressed three-dimensional geometry data may beimplemented in hardware, software, or a combination of each. Thedecompression unit includes an input FIFO that receives compressed databits and a signal noting size of the incoming data. The FIFO outputs arecoupled to an input block state machine and an input block. Outputs fromthe input block and input block state machine are coupled to a barrelshifter unit. Input block output also is input to Huffman tables thatoutput to the state machine. The state machine output also is coupled toa data path controller whose output is coupled to a tag decoder, and toa normal processor receiving output from the barrel shifter unit. Thedecompressor unit also includes a position/color processor that receivesoutput from the barrel shifter unit. Outputs from the normal processorand position/color processor are multiplexed to a format converter.

For instructions in the data stream that generate output to the formatconverter, the decompression unit generates a 12-bit tag that is sent tothe tag decoder in parallel with bits for normals that are sent to theformat converter. A read-back path is used to read back the internalstate of the decompressor unit. The decompressor unit carries out thefollowing procedures:

(1) Fetch the rest of the next instruction, and the first 8 bits of thefollowing instruction;

(2) Using the tag table, expand any compressed value fields to fullprecision;

(3A) If values are relative, add to current value; otherwise replace;

(3B) If mesh buffer reference, access old values;

(3C) If other command, do housekeeping;

(4) If normal, pass index through ROM table to obtain full values;

(5) Output values in generalized triangle strip form to next stage.

The decompressed stream of triangle data may then be passed to atraditional rendering pipeline, where it can be processed in fullfloating point accuracy, and thereafter displayed or otherwise used.

Other features and advantages of the invention will appear from thefollowing description in which the preferred embodiments have been setforth in detail, in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a generalized network system over which compressedthree-dimensional geometry may be transmitted for decompression,according to the present invention, at the receiving end;

FIG. 2 depicts a generalized triangular mesh data structure, andgeneralized mesh buffer representation of surface geometry;

FIG. 3 depicts six-way sign-bit and eight-way octant symmetry in a unitsphere, used to provide forty-eight way reduction in table look-up size;

FIG. 4A depicts a vertex command in a geometry compression instructionset;

FIG. 4B depicts a normal command in a geometry compression instructionset;

FIG. 4C depicts a color command in a geometry compression instructionset;

FIG. 4D depicts a mesh buffer reference command in a geometrycompression instruction set;

FIG. 4E depicts a set state instruction in a geometry compressioninstruction set;

FIG. 4F depicts a set table command instruction in a geometrycompression instruction set;

FIG. 4G depicts a pass through command instruction in a geometrycompression instruction set;

FIG. 4H depicts a variable length no-op command instruction in ageometry compression instruction set;

FIG. 4I depicts tag and Δ-position data structure;

FIGS. 4J-1 and 4J-2 depict alternative tag and Δ-normal data structure;

FIG. 4K depicts tag and Δ-color data structure;

FIG. 5 is a flowchart of method steps in a geometry compressionalgorithm;

FIG. 6 is a simplified block diagram of decompressor hardware, accordingto the present invention;

FIG. 7 is a detailed overall block diagram of a decompressor unit,according to the present invention;

FIG. 8 is a detailed block diagram of the input block shown in FIG. 7;

FIG. 9 is a detailed block diagram of the barrel shifter unit shown inFIG. 7;

FIG. 10 is a detailed block diagram of the position/color processor unitshown in FIG. 7;

FIG. 11A is a detailed block diagram of the normal processor unit shownin FIG. 7;

FIG. 11B is a detailed block diagram showing the decoder, fold, and ROMlook-up components associated with the normal processor unit of FIG.11A;

FIG. 12 is a block diagram showing interfaces to a mesh buffer, as shownin FIG. 10 and/or FIG. 11A;

FIG. 13A depicts interfaces to Huffman tables, according to the presentinvention;

FIG. 13B depicts a preferred format for entry of the Huffman table data,according to the present invention;

FIG. 14A depicts a vertex instruction, according to the presentinvention;

FIG. 14B depicts vertex component data formats, according to the presentinvention;

FIG. 14C depicts the format for the set normal instruction, according tothe present invention;

FIG. 14D depicts a set color instruction, according to the presentinvention;

FIG. 14E depicts a mesh buffer reference instruction, according to thepresent invention;

FIG. 14F depicts a set state instruction, according to the presentinvention;

FIG. 14G depicts a set table instruction, according to the presentinvention;

FIG. 14H depicts a passthrough instruction, according to the presentinvention;

FIG. 14I depicts a variable-length NOP instruction, according to thepresent invention; and

FIG. 14J depicts a skip 8 instruction, according to the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A graphics decompressor according to the present invention decompressesthree-dimensional graphics objects. Three-dimensional compression ofsuch geometry advantageously permits a reduction in the time needed totransmit the compressed three-dimensional geometry, e.g., over anetwork, as well a reduction of the space wherein the geometry may bestored, e.g., on a CD-ROM, or the like.

Before describing decompression of compressed three-dimensionalgraphics, the overall environment in which the present invention may bepracticed will be described with respect to FIG. 1.

FIG. 1 depicts a generalized network over which three-dimensionalcompressed geometry data may be transmitted, and decompressed usingsoftware, hardware, or a combination of each at the receiving end. Ofcourse, decompression of three-dimensional graphics compressionaccording to the present invention may be practiced upon compressed datathat is presented other than via a network, e.g., compressed data storedin a memory, on a CD-ROM, and the like.

As shown in FIG. 1, a source of three-dimensional graphics data 10 maybe coupled to a server or encoder system 20 whose processed andcompressed output is coupled over one or more networks 30 to one or moretarget clients or decoder systems 40. The network may be homogeneous,heterogeneous, or point-to-point. Server 20 includes a centralprocessing unit 50 that includes a central processor unit per se ("CPU")60 with associated main memory 70, a mesh buffer 80, a memory portion 90that may include a compression algorithm, and a region ofread-only-memory ("ROM") 100. Alternatively, compression according maybe carried out in hardware as opposed to software. ATTACHMENT 1 is acopy of a code listing for such a compression algorithm as described inthe above-referenced patent application. Server 20 also includes athree-dimensional graphics compression unit 60, whose compressed outputdata is arranged by a disk layout unit 70 for storage onto storage diskunit 80, which may include one or more CD-ROMs. The server communicatesover the network(s) 30 via network interface unit 110. Those skilled inthe art will appreciate that server 20 may include a mechanism forarbitrating between a plurality of client-decoder requests forcompressed data.

As described in applicant's patent application Ser. No. 08/511,294 filedAug. 4, 1995, entitled METHOD AND APPARATUS FOR GEOMETRIC COMPRESSION OFTHREE-DIMENSIONAL GRAPHICS DATA, assigned to the assignee herein, lossycompression of three-dimensional geometric data can produce ratios of6:1 to 10:1, with little loss in displayed object quality. The followingportions of this Specification will describe compression, as set forthin the above-referenced patent application, to facilitate a betterunderstand of decompression, according to the present invention.

In a network environment, at the receiving end, decoder systems(s) 40include a network interface unit 120, a unit 130, according to thepresent invention, that decompresses three-dimensional graphics data,and whose output is coupled to a three-dimensional graphics renderingunit 140. System 40 further comprises a central processing system 150that includes a CPU 160, memory 170, a portion of which 180 may includedecompression software, and ROM 190. Compressed three-dimensionalgraphics may advantageously be decompressed using software, hardware, ora combination of each. The decompressed output from decoder 40 furthermay be coupled to a viewer 200, or to another system requiring thedecompressed graphics. Of course, unit 40 may be a standalone unit, intowhich precompressed three-dimensional graphics are coupled from storage82, disks or CD-ROM 84, or the like, for decompression. Unit 40 may, forexample, comprise a computer or workstation.

Assuming that three-dimensional graphics compression unit 60 functionsas described in applicant's above-noted patent application, triangledata will have first been converted into a generalized triangle mesh.For a given fixed capacity of storage medium 80, a triangle mesh datastructure is a near-optimal representation of triangle data. In thepreferred embodiment, three-dimensional graphics object may berepresented as three-dimensional triangular data, whose format afterconversion causes each linear strip vertex, on average, to specify fromabout 1/3 triangles to about 2 triangles. Further, such triangle stripstructure permits extraction of the compressed geometry by a singlemonotonic scan over the vertex array data structure.

FIG. 2 depicts a generalized triangular mesh data structure, andgeneralized mesh buffer representation of surface geometry. Such a meshdata structure may be used in three-dimensional geometry compression,although by confining itself to linear strips, a generalized trianglestrip format wastes a potential factor of two in space. The geometryshown in FIG. 2, for example, can be represented by one triangle strip,but many interior vertices will appear twice in the strip.

In FIG. 2, a generalized triangle strip may be defined as follows, wherethe R denotes restart, O denotes replace oldest, M denotes replacemiddle, and a trailing letter p denotes push into mesh buffer. Thenumber following a capital letter is a vertex number, and a negativenumber is the mesh buffer reference, in which -1 denotes the most recentpushed vertex.

R6, O1, O7, O2, O3, M4, M8, O5, O9, O10, M11

M17, M16, M9, O15, O8, O7, M14, O13, M6,

O12, M18, M19, M20, M14, O21, O15, O22, O16,

O23, O17, O24, M30, M29, M28, M22, O21, M20,

M27, O26, M19, O25, O18

Using the same nomenclature, a generalized triangle mesh may be definedas follows:

R6p, O1, O7p, O2, O3, M4, M8p, O5, O9p, O10, M11,

M17p, M16p, M-3, O15p, O-5, O6, M14p, O13p, M9,

O12, M18p, M19p, M20p, M-5, O21p, O-7, O22p, O-9,

O23, O-10, O-7, M30, M29, M28, M-1, O-2, M-3, M27,

O26, M-4, O25, O-5

It is to be noted that a vertex reference advantageously can beconsiderably more compact (e.g., be represented by fewer bits) than afull vertex specification.

Three-dimensional geometry compression explicitly pushes old vertices(e.g., vertices with a trailing letter "p" above) into a queueassociated with mesh buffer memory 80 (see FIG. 1). These old verticeswill later be explicitly referenced when the old vertex is desiredagain. This approach provides a fine control that supports irregularmeshes of nearly any shape. Buffer memory 80 has finite length, and inpractice a maximum fixed queue length of 16 is used, which requires a4-bit index. With respect to the compression of three-dimensionalgraphics, the term "mesh buffer" shall refer to this queue, and theexpression "generalized triangle mesh" will refer to a combination ofgeneralized triangle strips and mesh buffer references.

The fixed size of mesh buffer 80 requires all tessellators/re-strippersfor compressed geometry to break-up any runs longer than sixteen uniquereferences. However, as geometry compression typically will not beprogrammed directly at the user level but rather by sophisticatedtessellators/reformatters, a non-onerous restriction. Sixteen oldvertices can in fact permit avoiding re-specification of up to about 94%of the redundant geometry.

FIG. 2 also is an example of a general mesh buffer representation ofsurface geometry. Geometry compression language supports the four vertexreplacement codes of generalized triangle strips, namely: replaceoldest, replace middle, restart clockwise, and restart counterclockwise.Further, the language adds an additional bit in each vertex header toindicate whether or not this vertex should be pushed into the meshbuffer. In one embodiment, the mesh buffer reference command has a 4-bitfield to indicate which old vertex should be re-referenced, along withthe 2-bit vertex replacement code. Mesh buffer reference commands do notcontain a mesh buffer push bit; old vertices can only be recycled once.

In practice, geometry rarely is comprised purely of positional data andin general, a normal, and/or color, and/or texture map coordinate arealso specified per vertex. Accordingly, entries into mesh buffer 80contain storage for all associated per-vertex information, specificallyincluding normal and color and/or texture map coordinate.

For maximum storage space efficiency, when a vertex is specified in thedata stream, per vertex normal and/or color information preferably isdirectly bundled with the position information. Preferably, suchbundling is controlled by two state bits: bundle normals with vertices(BNV), and bundle colors with vertices (BCV). FIG. 4E depicts a commandstructure including bits, among others. When a vertex is pushed into themesh buffer, these bits control if its bundled normal and/or color arepushed as well.

It should be noted that the compression technique described in theabove-referenced patent application is not limited to triangles, andthat vectors and dots may also be compressed. Lines, for example, are asubset of triangles, in which replacement bits are MOVE and DRAW. Anoutput vertex is then a vertex that represents one end point of a linewhose other vertex is the most recently, previously omitted vertex. Fordots, the replacement bits are DRAW, and an output vertex is the vertex.

When CPU 52 executes a mesh buffer reference command, this process isreversed. That is, the two bits specify whether a normal and/or colorshould be inherited, or read, from the mesh buffer storage 80, orobtained from the current normal or current color. Software 58preferably includes explicit commands for setting these two currentvalues. An exception to this rule exists, however, when an explicit "setcurrent normal" command is followed by a mesh buffer reference, with theBNV state bit active. In this situation, the former overrides the meshbuffer normal, to allow compact representation of hard edges in surfacegeometry. Analogous semantics are also defined for colors, allowingcompact representation of hard edges in surface colors.

Two additional state bits control the interpretation of normals andcolors when the stream of vertices is converted into triangles. Areplicate normals over triangle (RNT) bit indicates that the normal inthe final vertex that completes a triangle should be replicated over theentire triangle. A replicate colors over triangle (RCT) bit is definedanalogously, as shown in the command structure of FIG. 4E.

Compression of image xyz positions will now be described. Use of the8-bit exponent associated with 32-bit IEEE floating-point numbers allowspositions to range in size from sub-atomic particles to billions oflight years. But for any given tessellated object, the exponent isactually specified just once by a current modeling matrix, and objectgeometry is effectively described within a given modeling space usingonly a 24-bit fixed-point mantissa. In many cases far fewer bits areneeded for visual acceptance, and the geometry compression languagepreferably supports variable quantization of position data down to onebit.

At the other extreme, empirical visual tests as well as well asconsideration of semiconductor hardware implementation indicate that nomore than 16 bits of precision per component of position is necessaryfor nearly all cases.

Assume, however, that the position and scale of local modeling space perobject are specified by full 32-bit or 64-bit floating-pointcoordinates. Using sufficient numerical care, multiple such modelingspaces may be combined together to form seamless geometry coordinatesystems with much greater than 16-bit positional precision.

Most geometry is local. Thus, within a 16-bit (or less) modeling spacefor each object, the difference (Δ) between adjacent vertices in thegeneralized mesh buffer stream is likely to be less than 16 bits insignificance. If desired, one may construct a histogram representing bitlength of neighboring position Δ's in a batch of geometry, and basedupon this histogram assign a variable length code to compactly representthe vertices. As will be described, preferably customized Huffman codingis used to encode for the positional Δ's in the geometry compression.

Compression of red-blue-green-alpha ("RBGA") colors will now bedescribed. Color data are treated similarly to positions, but with asmaller maximum accuracy. Thus, RGBΔ color data are first quantized to12-bit unsigned fraction components that are absolute linearreflectivity values (in which 1.0 represents 100% reflectivity). Anadditional parameter allows color data effectively to be quantized toany amount less than 12 bits. By way of example, colors may all bewithin a 5-5-5 RGB color space, as shown in FIG. 4C. The optional Δfield is controlled by a color Δ present ("CAP") state bit shown in FIG.4E. On the final rendered image individual pixel colors are stillinterpolated between the quantized vertex colors, and also typically aresubject to lighting.

In practice, the same A-coding may be used for color components andpositions. The area of color data compression is where geometrycompression and traditional image compression confront the most similarproblems. However, many advanced image compression techniques may beavoided for geometry color compression because of the difference infocus.

For example, the JPEG image compression standard relies upon assumptionsabout viewing of the decompressed data that cannot be made for geometrycompression. For example, in image compression, it is known a priorithat the pixels appear in a perfectly rectangular array, and that whenviewed, each pixel subtends a narrow range of visual angles. Bycontrast, in geometry compression, the relationship between the viewerand the rasterized geometry is unpredictable.

In image compression, it is known that the spatial frequency of thedisplayed pixels upon on the viewer's eyes is likely higher than thecolor acuity of the human visual system. For this reason, colors arecommonly converted to YUV space so that the UV color components can berepresented at a lower spatial frequency than the Y (intensity)component. Usually digital bits representing sub-sampled UV componentsare divided among two or more pixels. However, geometry compressioncannot take advantage of this because there is no fixed display scale ofthe geometry relative to the viewer's eye. Further, given thatcompressed triangle vertices are connected to four to eight or moreother vertices in the generalized triangle mesh, there is no consistentway of sharing "half" the color information across vertices.

Similar arguments apply for the more sophisticated transforms used intraditional image compression, such as the discrete cosine transform.These transforms assume a regular (rectangular) sampling of pixelvalues, and require a large amount of random access duringdecompression.

It is known in the art to use pseudo-color look-up tables, but suchtables would required a fixed maximum size, and would represent arelatively expensive resource for real-time processing. Whilepseudo-color indices could yield slightly higher compression ratios forcertain scenes, the RGB model is more generalized and considerably lessexpensive to implement.

In an RGB model, RGB values are represented as linear reflectancevalues.

Theoretically, if all effects of lighting could be known a priori, oneor two representation bits could be dropped if the RGB components hadbeen represented in a nonlinear, or perceptually linear space (sometimereferred to as gamma corrected space). In practice, lighting effectstend not to be predictable, and on-the-fly conversion from nonlinearlight to linear light would require considerable hardware resources.

The compression of surface normals will now be described. Traditionally96-bit normals (three 32-bit IEEE floating-point numbers) are used incalculations to determine 8-bit color intensities. Theoretically, 96bits of in formation could be used to represent 2⁹⁶ different normals,spread evenly over the surface of a unit sphere. The resultant extremelyhigh accuracy represents a normal projecting in any direction every 2⁻⁴⁶radians.

But for IEEE floating-point normalized normals, the exponent bits areeffectively unused. Given the constraint N_(x) ² +N_(y) ² +N_(z) ² =1,at least one of N_(x), N_(y), or N_(z) must be in the 0.5 to 1.0 range.During rendering, this normal will be transformed by a compositemodeling orientation matrix:

    N'.sub.x =N.sub.x ·T.sub.O,O +N.sub.y ·T.sub.O,1 +N.sub.z ·T.sub.O,2

    N'.sub.y =N.sub.x ·T.sub.1,O +N.sub.y ·T.sub.1,1 +N.sub.z ·T.sub.1,2

    N'.sub.z =N.sub.x ·T.sub.2,O +N.sub.y ·T.sub.2,1 +N.sub.z ·T.sub.2,2

Assuming a typical implementation in which lighting is performed inworld coordinates, the view transform is not involved in the processingof normals. If the normals have been pre-normalized, then to avoidredundant re-normalization of the normals, the composite modelingtransformation matrix T is typically pre-normalized to divide out anyscale changes. Thus:

    T.sub.0,0.sup.2 +T.sub.1,0.sup.2 +T.sub.2,0.sup.2 =1, etc.

During normal transformation, floating-point arithmetic hardwareeffectively truncates all additive arguments to the accuracy of thelargest component. The result is that for a normalized normal undergoingtransformation by a scale preserving modeling orientation matrix, thenumerical accuracy of the transformed normal value is reduced to no morethan 24-bit fixed-point accuracy in all but a few special cases.

By comparison, even 24-bit normal components would still provide higherangular accuracy than the repaired Hubble space telescope, and inpractice, some systems utilize only 16-bit normal components. Inempirical tests with 16-bit normal components, results from an angulardensity of 0.01 radians between normals (e.g., about 100,000 normalsdistributed over a unit sphere) are not visually distinguishable fromfiner representations. In rectilinear space, these normals still requirehigh representation accuracy and in practice, 16-bit componentsincluding one sign and one guard bit represents a good design choice.This still requires 48 bits to represent a normal, but since only100,000 specific normals are of interest, theoretically a single 17-bitindex could denote any of these normals.

The use of normals as indices, and the resultant advantages providedwill now be described. One method of converting an index of a normal onthe unit sphere back into a N_(x), N_(y), N_(z) value is with a tablelook-up, the table being loaded into memory 70 perhaps. Although tablesize is potentially large, the requisite size can be substantiallyreduced by taking advantage of a 48-way symmetry present in the unitsphere.

More particularly, as shown by FIG. 3, the unit sphere is symmetrical bysign bits in the eight quadrants by sign bits. By allowing three of thenormal representation bits to be the three sign bits of the xyzcomponents of a normal, it then is only necessary to represent oneeighth of the unit sphere. Each octant of the unit sphere can be dividedinto six identical components by folding about the planes x=y, x=z, andy=z. The six possible sextants are encoded with another three bits,which leaves only 1/48 of the sphere remains to be represented.

Utilizing the above-noted symmetry reduces the look-up table size by afactor of 8×6=48. Instead of storing 100,000 entries, the look-up tableneed store only about 2,000 entries, a size small enough to be anon-chip ROM look-up table, stored perhaps within ROM 59 (see FIG. 1).Indexing into the look-up table requires 11 address bits, which whenadded to the previously described two 3-bit fields results in a 17-bitfield to describe all three normal components.

Representing a finite set of unit normals is equivalent to positioningpoints on the surface of the unit sphere. Although no perfectly equalangular density distribution exists for large numbers of points, manynear-optimal distributions exist. Theoretically, a distribution havingthe above-described type of 48-way symmetry could be used for thedecompression look-up table associated with the three-dimensionalgeometry decompression unit 130 (see FIG. 1).

However, several additional constraints mandate a different choice ofencoding. First, a scalable density distribution is desired, e.g., adistribution in which setting in the look-up table more low orderaddress bits to "0" still results in fairly even normal density on theunit sphere. Otherwise a different look-up table for every encodingdensity would be required. Secondly, a Δ-encodable distribution isdesired in that adjacent vertices in geometry statistically have normalsthat are nearby on the surface of the unit sphere. Nearby locations onthe two-dimensional space of the unit-sphere surface are most succinctlyencoded by a two-dimensional offset. It is desirable to have adistribution in which such a metric exists. Finally, althoughcomputational costs associated with the normal encoding process are notcritically important, distributions having lower encoding costs arestill preferred.

Compression according to the above-referenced patent applicationutilizes a distribution having a regular grid in the angular spacewithin one sextant of the unit sphere. As such, rather than a monolithic11-bit index, all normals within a sextant are advantageouslyrepresented with two 6-bit orthogonal angular addresses. Thisconfiguration then revises the previous bit-total to 18-bits. As was thecase for positions and colors, if more quantization of normals isacceptable, these 6-bit indices can be reduced to fewer bits, and thusabsolute normals can be represented using anywhere from 18 to as few as6 bits. However, as described below, this space preferably is Δ-encodedto further reducing the number of bits required for high qualityrepresentation of normals.

Normal encoding parameterization will now be described. Points on a unitradius sphere are parameterized using spherical coordinates by angles θand φ, where θ is the angle about the y axis and φ is the longitudinalangle from the y=0 plane. Equation (1) governs mapping betweenrectangular and spherical coordinates as follows:

    x=cosθcosφy=sinφz=sinθcosφ         (1)

Points on the sphere are folded first by octant, and then by sort orderof xyz into one of six sextants. All table encoding takes place in thepositive octant in the region bounded by the half spaces:

    x≧zz≧yy≧0

As shown in FIG. 3, the described triangular-shaped patch runs from 0 toπ/4 radians in θ, and from 0 to a maximum 0.615479709 radians in φ.

Quantized angles are represented by two n-bit integers θ_(n) and θ_(n),where n is in the range of 0 to 6. For a given n, the relationshipbetween indices θ and φ is: ##EQU1##

Equations (2) show how values of θ_(n) and φ_(n) can be converted tospherical coordinates θ and φ, which in turn can be converted torectilinear normal coordinate components via equation (1).

To reverse the process, e.g. to encode a given normal N into θ_(n) andφ_(n), one cannot simply invert equation (2). Instead, the N must befirst folded into the canonical octant and sextant, resulting in N'.Then N' must be dotted with all quantized normals in the sextant. For afixed n, the values of θ_(n) and φ_(n) that result in the largest(nearest unity) dot product define the proper encoding of N. Other, moreefficient methods for finding the correct values of θ_(n) and φ_(n)exist, for example indexing through the table to set φ, and then jumpinginto θ.

At this juncture, the complete bit format of absolute normals can begiven. The uppermost three bits specify the octant, the next three bitsthe sextant, and finally two n-bit fields specify θ_(n) and φ_(n). The3-bit sextant field takes on one of six values, the binary codes forwhich are shown in FIG. 3.

Some further details are in order. The three normals at the corners ofthe canonical patch are multiply represented, namely 6, 8, and 12 times.By employing the two unused values of the sextant field, these normalscan be uniquely encoded as 26 special normals.

This representation of normals is amenable to Δ-encoding, at leastwithin a sextant, although with some additional work, this can beextended to sextants that share a common edge. The Δ code between twonormals is simply the difference in θ_(n) and φ_(n), namely Δθ_(n) andΔφ_(n).

In the above-described patent application, compression tags are used,with a variation of a conventional Huffman algorithm. The Huffmancompression algorithm takes in a set of symbols to be represented, alongwith frequency of occurrence statistics (e.g., histograms) of thosesymbols. From this, variable length, uniquely identifiable bit patternsare generated that allow these symbols to be represent ed with anear-minimum total number of bits, assuming that symbols do occur at thefrequencies specified.

Many compression techniques, including JPEG, create unique symbols astags to indicate the length of a variable-length data-field thatfollows. This data field is typically a specific-length delta value.Thus, the final binary stream consists of (self-describing length)variable length tag symbols, each immediately followed by a data fieldwhose length is associated with that unique tag symbol.

In the referenced patent application, binary format for geometrycompression uses this technique to represent position, normal, and colordata fields. For geometry compression, these <tag, data> fields areimmediately preceded by a more conventional computer instruction setop-code field. These fields, along with potential additional operandbits, will be referred to as geometry instructions (see FIGS. 4A-4K).

Traditionally, each value to be compressed is assigned its ownassociated label, e.g. an xyz Δ position would be represented by threetag-value pairs. But since the Δxyz values are not uncorrelated, adenser, simpler representation can be attained. In general, the xyz Δ'sstatistically point equally in all directions in space. Thus, if n isthe number of bits needed to represent the largest of the Δ's, thenstatistically the other two Δ values require an average of n-1.4 bitsfor their representation. In practice, a single field-length tag may beused to indicate the bit length of Δx, Δy, and Δz.

Unfortunately, using this approach prevents taking advantage of anotherHuffman technique to save somewhat less than one more bit per component.However, the implemented embodiment outweighs this disadvantage by nothaving to specify two additional tag fields (for Δy and Δz). A furtheradvantage is that using a single tag field permits a hardwaredecompression engine to decompress all three fields in parallel, ifdesired.

Similar arguments hold for Δ's of RGBα values, and accordingly a singlefield-length tag is used to indicate bit-length of the R, G, B and, ifpresent, α, fields.

Absolute and Δ normals are also parameterized by a single value (n) thatcan be specified by a single tag. To facilitate high-speed, low-costhardware implementations, the length of the Huffman tag field may belimited to six bits, a relatively small value. A 64-entry tag look-uptable allows decoding of tags in one clock cycle. One table exists forpositions, another table exists for normals, and yet another tableexists for colors (and optionally, also for texture coordinates). Eachtable contains the length of the tag field, the length of the datafield(s), a data normalization coefficient, and an absolute/relativebit.

For reasonable hardware implementation, an additional complication mustbe addressed. As described below, all instruction are broken-up into aneight-bit header, and a variable length body, sufficient informationbeing present in the header to determine the body length. But the headerof one instruction must be placed in the data stream before the body ofthe previous instruction to give the hardware time to process the headerinformation. For example, the sequence . . . B0 H1B1 H2B2 H3 . . . hasto be encoded as . . . H1 B0 H2 B1 H3 B2 . . . .

The geometry compression instruction set disclosed in theabove-referenced patent application will now be described with respectto FIGS. 4A-4K. FIG. 4A depicts a vertex command that specifies aHuffman compressed Δ-encoded position, as well as possibly a normaland/or color, depending on bundling bits (BNV and BCV). Two additionalbits specify a vertex replacement code (REP), and another bit controlsmesh buffer pushing of this vertex (MBP).

As shown in FIG. 4B, a normal command specifies a new current normal andthe color command shown in FIG. 4C depicts a new current color. Thenormal command and color command each use Huffman encoding of Δ values.

The mesh buffer reference command structure is shown in FIG. 4D. Themesh buffer reference command allows any of the sixteen most recentlypushed vertices (and associated normals and/or colors) to be referencedas the next vertex. As further shown in FIG. 4D, A 2-bit vertexreplacement ("REP") code is also specified.

FIG. 4E depicts the set state instruction that updates the five statebits: RNT, RCT, BNV, BCV, and CAP.

FIG. 4F depicts a set table command, which is used to set entries to theentry value specified in one of the three Huffman decoding tables(Position, Normal, or Color).

FIG. 4G depicts a passthrough command that allows additional graphicsstate not controlled directly by geometry compression to be updatedin-line.

FIG. 4H depicts a variable length no-op ("VNOP") command that allowsfields within the bit stream to be aligned to 32-bit word boundaries.This permits aligned fields to be efficiently patched at run-time by thegeneral CPU 52.

FIGS. 4I, 4J-1 and 4J-2 and 4K respectively depict tag and Δ-positiondata structure, tag and A-normal data structure, and tag and Δ-colordata structure. In FIGS. 4I and 4K, either absolute values of x, y, zare used, or delta values of x, y, and z are to be used.

Of course, other instruction sets may instead be used to compressthree-dimensional geometry.

The ratio of the time required for compression relative to decompressioncan be important. In practice, it is acceptable for off-line imagecompression to take up to perhaps sixty-times more time thandecompression, but for real-time video conferencing, the ratio should beone.

Advantageously, geometry compression does not have this real-timerequirement. Even if geometry is constructed on the fly, most geometrycreating techniques, e.g., CSG, require orders of magnitude more timethan needed for displaying geometry. Also, unlike continuous imagesfound in movies, in most applications of geometry compression acompressed three-dimensional object will be displayed for manysequential frames before being discarded. Should the three-dimensionalobject require animating, animation is typically done with modelingmatrices. Indeed for a CD-based game, it is quite likely that an objectwill be decompressed billions of times by customer-users, but will havebeen compressed only once by the authoring company.

Like some other compression systems, geometry compression algorithms canhave a compression-time vs. compression-ratio trade-off. For a givenquality target level, as allowable time for compression increases, thecompression ratio achieved by a geometry compression system increases.There exists a corresponding "knob" for quality of the resultingcompressed three-dimensional object, and lower the quality knob, thebetter the compression ratio achieved.

Aesthetic and subjective judgment may be applied to geometrycompression. Some three-dimensional objects will begin to appear badwhen target quantization of normals and/or positions is slightlyreduced, whereas other objects may be visually unchanged even with alarge amount of quantization. Compression can sometimes cause visibleartifacts, but in other cases may only make the object look different,not necessarily lower in quality. In one experiment by applicant, animage of an elephant actually begin to appear more realistic, with morewrinkle-like skin, as the image normals were quantized more. Once amodel has been created and compressed, it can be put into a library, tobe used as three-dimensional clip-art at the system level.

While many aspects of geometry compression are universal, theabove-described geometry compression instruction set has been somewhattailored to permit low-cost, high-speed hardware implementations. (It isunderstood that a geometry compression format designed purely forsoftware decompression would be somewhat different.). The describedgeometry compression instruction set is especially amenable to hardwareimplementation because of the one-pass sequential processing, limitedlocal storage requirements, tag look-up (as opposed to a conventionalHamming bit-sequential processing), and use of shifts, adds, andlook-ups to accomplish most arithmetic steps.

FIG. 5 is a flowchart outlining method steps in a geometry compressionalgorithm routine, described in the above-referenced patent application,with which the present decompression invention may be used. Such routinemay be stored in memory 80 and executed under control of CPU 60 (seeFIG. 1).

At step 200, an object is represented by an explicit group of trianglesto be compressed, along with quantization thresholds for positions,normals, and colors. At step 210, a topological analysis of connectivityis made, and hard edges are marked in normals and/or color, if suchinformation is not already present.

At step 220, vertex traversal order and mesh buffer references arecreated, and at step 230 histograms of Δ-positions, Δ-normals, andΔ-colors is created. At step 240, separate variable length Huffman tagcodes are assigned for the Δ-positions, Δ-normals, and Δ-colors, basedupon histographs.

At step 250, a binary output stream is generated by first outputtingHuffman table initialization, after which the vertices are traversed inorder. Appropriate tags and Δ's are output for all values.

Applicant has implemented a Wavefront OBJ format compressor thatsupports compression of positions and normals, and creates fullgeneralized triangle strips, but does not yet implement a fullmeshifying algorithm. Future embodiments will explore variable precisiongeometry, including fine structured updates of the compression tables.The current compressor expends time calculating geometric detailsalready known to the tessellator, and ultimately it is hoped to generatecompressed geometry directly. However, even its present unoptimizedstate, applicant's software can compress about 3,000 triangles/second inmany cases.

The present invention is directed to decompressing three-dimensionalcompressed geometry, at the user end of FIG. 1. ATTACHMENT 2 is alisting of an algorithm for decompression, according to the presentinvention. Briefly, in general, an applicable geometry decompressionalgorithm according to the present invention may be outlined as follows:

(1) Fetch the rest of the next instruction, and the first 8 bits of thefollowing instruction;

(2) Using the tag table, expand any compressed value fields to fullprecision;

(3A) If values are relative, add to current value; otherwise replace;

(3B) If mesh buffer reference, access old values;

(3C) If other command, do housekeeping.

(4) If normal, pass index through ROM table to obtain full values.

(5) Output values in generalized triangle strip form to next stage.

In the preferred embodiment, a software embodiment of applicant'sdecompressor decompresses compressed geometry at a rate of about 10,000triangles/second. A simplified overall block diagram of decompressionaccording to the present invention is shown in FIG. 6. A hardwareimplementation of a decompressor according to the present invention candecompress in the range of tens of millions of triangles/second, whichrate may be substantially expanded.

Before describing decompression, it is helpful to examine the results ofthe above-described compression techniques. Table 1, shown below,describes these results for several graphical objects: a triceratops, aSpanish galleon, a Dodge Viper, a '57 Chevy, and an insect. Generallyspeaking, Table 1 shows that positional quantization much above 24 bits(from an original 32 bits per x/y/z coordinate) has no significantvisible effects unless zooming is performed on the object. Positionalquantization to 24 bits is denoted herein as "P72" (24×3). Furthermore,normal coordinates may be reduced from 96 bits (32 bits per coordinate)to as little as 36 bits (12 bits per coordinate) with little visiblechange. Normal quantization to 12 bits per coordinate is denoted hereinas "N36" (12×3). While the location of specular highlights may differslightly with normal quantization, it is not visually apparent that suchchanges are reductions in quality.

Table 1 summarizes compression and other statistics for these objects.Column 1 notes the object in question, column 2 represents the number ofΔ's, and column three the Δ-strip length. The fourth column representssystem overhead per vertex (overhead being everything beyond positiontag/data, and normal tag/data). The "xyz quant" column denotesquantization thresholds, and the sixth column depicts the number ofbits/xyz. "Bits/tri" ninth column depicts bits per triangle.

The results in Table 1 are measured actual compression data except forestimated mesh buffer results, which are shown in parenthesis. No actualmesh buffer results were present in that applicant's prototype softwarecompressor did not yet implement a full meshifying algorithm. Theestimate (in parenthesis) assumes a 46% hit ratio in the mesh buffer.

In Table 1, the right-most column shows compression ratio performanceachieved over existing executable geometry formats. Although total bytecount of the compressed geometry is an unambiguous number, in stating acompression ratio some assumptions must be made about the uncompressedexecutable representation of the object. Applicant assumed optimizedgeneralized triangle strips, with both positions and normals representedby floating-point values to calculate "original size" data for Table 1.

To demonstrate the effect of pure 16-bit fixed point simple striprepresentation, Table 1 also shows byte count for the mode of OpenGL. Asshown, average strip length decreased in the range of 2-3. Few if anycommercial products take advantage of generalized triangle strips, andthus Table 1 considerably understates potential memory space savings.

                                      TABLE 1                                     __________________________________________________________________________    Obj-    Δstp                                                                       ovrhd/                                                                            xyz                                                                              bits/                                                                            norm                                                                             bits/                                                                            bits/                                                                             org'l size                                                                         comp. size                                                                          comp.                               name                                                                              #Δ's                                                                        len.                                                                             vertex                                                                            quant                                                                            xyz                                                                              quant                                                                            norm                                                                             tri (bytes)                                                                            (bytes)                                                                             ratio                               __________________________________________________________________________    tricer-                                                                           6,039                                                                             15.9                                                                             7.5 48 30.8                                                                             18 16.8                                                                             55.9                                                                              179,704                                                                            42,190                                                                              4.3X                                atops                      (35.0)   (26,380)                                                                            (6.9X)                              ticer-                                                                            6,039                                                                             15.9                                                                             7.5 30 17.8                                                                             12 11.0                                                                             36.0                                                                              179,704                                                                            27,159                                                                              6.7X                                atops                      (24.4)   (18,368)                                                                            (9.8X)                              galleon                                                                           5,577                                                                             12.1                                                                             7.5 30 21.9                                                                             12 10.8                                                                             41.0                                                                              169,064                                                                            28,536                                                                              6.0X                                                           (27.2)   (18,907)                                                                            (9.0X)                              Viper                                                                             58,203                                                                            23.8                                                                             7.5 36 20.1                                                                             14 10.9                                                                             37.5                                                                              1,698,116                                                                          272,130                                                                             6.3X                                                           (25.0)   (181,644)                                                                           (9.4X)                              57  31,762                                                                            12.9                                                                             7.5 33 17.3                                                                             13 10.9                                                                             35.8                                                                              958,160                                                                            141,830                                                                             6.8X                                Chevy                      (24.3)   (96,281)                                                                            (10.0X)                             insect                                                                            263,783                                                                           3.0                                                                              7.5 39 22.8                                                                             15 11.0                                                                             51.5                                                                              9,831,528                                                                          1,696,283                                                                           5.8X                                                           (33.9)   (1,115,534)                                                                         (8.9X)                              __________________________________________________________________________

While certainly statistical variation exists between objects withrespect to compression ratios, general trends are nonetheless noted.When compressing using the highest quality setting of the quantizationknobs (P48/N18), compression ratios are typically about six. As ratiosapproach nearly then, most objects begin to show visible quantizationartifacts.

It will be appreciated from the foregoing, that a three-dimensionalgeometry compression algorithm may be implemented in real-time hardware,or in software. Significantly, if three-dimensional rendering hardwarecontains a geometry decompression unit according to the presentinvention, application geometry may be stored in memory in compressedformat. Further, data transmission may use the compressed format, thusimproving effective bandwidth for a graphics accelerator system,including shared virtual reality display environments. The resultantcompression can substantially increase the amount of geometry cacheablein main memory.

FIG. 7 is a detailed block diagram of the decompressor unit 130, shownin FIG. 1. As shown in FIG. 7, unit 130 includes a decompression inputfirst-in-first-out register ("FIFO") 200 whose inputs include controlsignals and a preferably 32-bit or 64-bit data stream, which signals anddata stream preferably come from an accelerator port data FIFO ("APDF")in interface unit 120 (see FIG. 1). The APDD portion of interface 120includes a controller that signals the size of the incoming data streamto unit 130. FIFO 200 provides output to an input block state machine220 and to an input block 210, state machine 220 and input block unit210 communicating with each other.

Output from block 210 is coupled to a barrel shifter unit 240 and to aHuffman table set 230, the output from the Huffman look-up being coupledto state machine 220. Opcode within state machine 220 processes thevalues provided by the Huffman tables 230 and outputs data to the barrelshifter unit 240. State machine 220 also provides an output to data pathcontroller 260, which outputs a preferably 12-bit wide signal to a tagdecoder unit 294 and also outputs data to the barrel shifter unit 240and to a normal processor 270, and a position/color processor 280.

Barrel shifter unit 240 outputs to the normal processor 270 and to aposition/color processor 280. The outputs from processors 270 and 280are multiplexed by output multiplexer unit 290 into a preferably 48-bitwide signal that is provided to a format converter 292. Decompressionunit 130 generates a preferably 12-bit tag that is sent to tag decoder294 in parallel with either 32-bits or 48-bits (for normals), that aresent to the format converter 292. These data streams provideinstructions that generate output to format converter 292. A preferably32-bit read-back path is used to read-back the state of the unit.

Table 2, below, shows interface signals used to implement decompressionunit 130 in the preferred embodiment:

                  TABLE 2                                                         ______________________________________                                        Signal Name Signals I/O      Description                                      ______________________________________                                        id.sub.-- data                                                                            64      I        Data inputs from                                                              APDF                                             id.sub.-- tag                                                                             12      I        Data on inputs is valid                                                       from APDF                                        fd.sub.-- stall                                                                           1       I        Stall signal from                                                             format converter                                 di.sub.-- busy                                                                            1       O        Busy signal to status                                                         register                                         di.sub.-- faf                                                                             1       O        Fifo-almost-full                                                              signal-to-input FIFO                             df.sub.-- data                                                                            48      O        Data output to formal                                                         converter                                        df.sub.-- tag                                                                             12      O        Tag output to tag                                                             decoder                                          du.sub.-- context                                                                         32      O        Context output to UPA                                                         section                                          ______________________________________                                    

Table 3, below, shows output data formats provided by unit 130 in thepreferred embodiment. As described herein, vertex, mesh bufferreference, and passthrough instructions generate transactions fromdecompression unit 130. Vertex and mesh buffer reference instructionssend data to the format converter, and each generates a headerindicating vertex replacement policy for the current vertex, followed bycomponent data. Each of these instructions always generates positiondata and, depending upon the value of the state register, may containcolor or normal data. All three of the normal components preferably aresent in parallel, whereas each position and color component isseparately sent. A passthrough instruction sends preferably 32-bits ofdata to the collection buffer.

                  TABLE 3                                                         ______________________________________                                        COMPONENTS            FORMAT                                                  ______________________________________                                        Header                32.                                                     Position              s.15                                                    Color                 s.15                                                    Normal                s1.14(×3)                                         Passthrough           32.                                                     ______________________________________                                    

FIG. 8 is a detailed block diagram of the input block 210 depicted inFIG. 7. A preferably 64-bit input register 300 receives data from theAPDF portion of interface 130, with 32-bits or 64-bits at a time beingloaded into register 300. Register 300 outputs preferably 32-bits at atime via multiplexer 310 to a first barrel shifter 320 whose outputpasses through a register 330 into a merge unit 340. The 64-bit outputfrom merge unit 340 is input to data register 350, part of whose outputis returned as input to a second barrel shifter 360. The output fromsecond barrel shifter 360 is passed through a register 370 and is alsoinput to merge unit 340. First barrel shifter 320 aligns data to thetail of the bit-aligned data stream being recycled from data register350 through second barrel shifter 360. The second barrel shifter 360shifts-off the used bits from data register 350.

FIG. 9 is a detailed block diagram of barrel shifter unit 240, shown inFIG. 7. In overview, barrel shifter unit 240 expands the variable-lengthposition, color, and normal index components to their fixed-pointprecisions. Data into unit 240 from unit 210 and/or 220 is input to aregister 400 whose output is shown as defining opcode and/or data units410, 420, 430, 440, 450, and 460, which are input to a multiplexer unit470.

Multiplexer unit 470 input A is used for the X component of the vertexinstruction, input B is used for the set normal instruction and thefirst component of the set color instructions, and input C is used forthe remaining components of the vertex and set color instructions. Unit240 further includes a barrel shift left register 480 coupled to receivetag₋₋ len data and to output to register 490, whose output in turn isinput to a barrel shift right register 500 that is coupled to receivedata₋₋ len data. Register 500 outputs to a mask unit 510 that is coupledto receive shift dfata and whose output is coupled to register 520,which outputs v₋₋ data. The output of data block 460 is coupled to aregister 530 whose output is coupled to a second register 540, whichoutputs pt₋₋ data.

An appropriate table within Huffman tables 230 (see FIG. 7) providesvalues of tag₋₋ len, data₋₋ len, and shift into units 480, 500 and 510,respectively. Barrel shift left unit 480 shifts the input data left by 0to 6 bits (tag len), thus shifting off the Huffman tag. By contrast,barrel shift right register 500 shifts the data to the right by 0 to 16bits (16-data₋₋ len), and sign extends the data, thus bringing the datato its full size. Mask unit 510 masks off the lower `shift` bits toclamp the data to the correct quantization level.

FIG. 10 depicts in greater block diagram detail the position/colorprocessor unit 280, shown in FIG. 7. Processor unit 280 generates finalposition or color component values. As shown in FIGS. 7 and 9, processorunit 280 receives a preferably 16-bit value (v₋₋ data) from the barrelshifter unit 240, specifically mask unit 510 therein.

If the abs₋₋ rel bit from the Huffman table 230 is set to relative, theincoming data are added by combiner unit 600 to the appropriate currentstored data. The new value passes through multiplexer 610, and is storedback into the register 620, and is sent along to the output multiplexer290, shown in FIG. 7. However, if the abs₋₋ rel bit is set to absolute,the incoming data bypasses adder 600, is latched into the register 620,and is also sent out to the output multiplexer 290.

As shown in FIG. 10, the position/color processor unit 280 furtherincludes a position/color mesh buffer 630 that is coupled to receive theinput to register 620. The output from mesh buffer 630 is coupled tomultiplexer gates, collectively 640, whose outputs reflect currentvalues of x, y, z, r, g, b and α. A register set, collectively shown as650, provides these current values to the input of a multiplexer 660,whose output is coupled to the adder 600. Processor unit 280 furtherincludes a register 670 that receives and outputs pt₋₋ data from barrelshifter unit 240.

As shown in FIG. 7, normal processor unit 270 also outputs data to theoutput multiplexer 290. FIG. 11A depicts in detail the sub-unitscomprising normal processor unit 270. As seen in FIG. 7 and FIG. 9, thenormal processor unit 270 receives an 18-bit normal index as threeseparate components: sextant/octant, u and v, or encoded Δu and Δvcomponents from mask unit 510 in barrel shifter unit 240. If the valueis a Δ-value (relative), the Δu and Δv are added to the current u and vvalues by respective adders 710. The intermediate values are stored andare also passed on to a fold unit 800 associated with decoder-fold-romunit 272 (see FIG. 11B).

As shown in FIG. 11A, the normal processor unit 270 further includesregisters 712, 714, 716, 718, 720, 722, 724, 726 which hold respectiveoctant, sextant, u and v values, curr₋₋ oct, curr₋₋ sext, curr₋₋ u andcurr₋₋ v values. Also present in unit 270 are multiplexers 740, 742,744, 746, 748, 750, 752, 754, 756, 758 and 760, 1's complementing units770, 772, latch-flipflop units 780, 782, 784 for holding respective v,u, and uv information, further adders 790, 792, and a normal mesh buffer794 coupled to receive curr₋₋ normal input components.

With reference to FIGS. 11A and 11B, for an absolute reference, the uand v values are passed directly to fold unit 800. The octant andsextant portions of the index are sent to decoder 810, within unit 272.Decoder 810 controls multiplexer 820 (which select constants), as wellas multiplexers 840, 842, 844, 860, 862, 864, which reorder components,and invert signs (using 2's complement units 850, 852, 854).

Fold unit 800 uses the u and v components of the normal index, from unit270, to calculate the address into the normal look-up table ROM 830. Theoctant and sextant fields, from unit 270, drive a decoder 810 thatdetermines sign and ordering of components output from the ROM look-uptable 830. Decoder 810 also handles special case normals not included inthe normal ROM look-up table 830.

FIG. 12 depicts interfaces to a mesh buffer, as shown in FIG. 10 and/orFIG. 11A. In the preferred embodiment, mesh buffer 794 is implemented asa register file and a pointer to the current location. Data is input tothe mesh buffer FIFO at the position of the current location pointer.However, random access to any of the 16 locations is allowed whenreading the data out of the FIFO by indexing off this pointer:address=(curr₋₋ loc₋₋ ptr--index) mod 16.

FIG. 13A depicts interfaces to Huffman tables, e.g., tables 230 in FIG.7. Huffman tables are used to decode the Huffman tags preceding thecompressed data. Three Huffman tables are used: one for position, forcolor, and for normal data, with each table preferably holding 64entries.

FIG. 13B depicts a preferred format for entry of position and color datain the Huffman tables, while FIG. 13C depicts the preferred format fornormal table entries. The instruction format for loading the Huffmantables in the compressed data stream is described later herein.

Several instructions generate data for the format converter 292, shownin FIG. 7, and appropriate tags must be generated for this data so theformat converter can correctly process the data. Table 4, below, showstags generated for the different data components.

The components that show two tags may set the launch bit, and the secondtag shows the value with the launch bit set.

                  TABLE 4                                                         ______________________________________                                        COMPONENTS           TAG                                                      ______________________________________                                        Header               0x020                                                    X                    0x011                                                    Y                    0x012                                                    Z                    0x013/0x413                                              Nx/Ny/Nz             0x018/0x418                                              R                    0x014                                                    G                    0x015                                                    B                    0x016/0x416                                              A                    0x017/0x417                                              U                    0x0c0/0x4c0                                              V                    0x01c/0x41c                                              ______________________________________                                    

Input block state machine 220 (see FIG. 7) includes a preferably six-bitstate register that holds information about the processing state of thedecompression unit. In the preferred embodiment, the following statebits are defined:

Bit 5: tex--Texture values in place of color

Bit 4: rnt--Replicate normal per vertex

Bit 3: rct--Replicate color per vertex

Bit 2: bnv--Normal bundled with vertex

Bit 1: bcv--Color bundled with vertex

Bit 0: cap--Color includes alpha (α)

Position/Color processor unit 280 (see FIGS. 7 and 10) preferablyincludes three 16-bit registers, curr₋₋ x, curr₋₋ y, and curr₋₋ z, whichcontain the current position components, X, Y, and Z, and are onlyupdated by vertex instructions.

Normal processor unit 270 (see FIGS. 7 and 11A) preferably includesthree six-bit registers, curr₋₋ oct, curr₋₋ sext, curr₋₋ u, curr₋₋ v)that contain the current normal. The first register holds the 3-bitsextant and octant fields, and the remaining two registers contain the uand v coordinates for the normal. These values are written using the setnormal instruction, or they are updated by the vertex instruction if thebnv bit is set in the state register.

Position/color processor 280 further preferably includes four 16-bitregisters, curr₋₋ r, curr₋₋ g, curr₋₋ b, curr₋₋ a, which contain thecurrent color components, red, green, blue and alpha (α). Thesecomponents are set using the se5t color instruction, or they are updatedby the vertex instruction if the bcv bit is set in the state register.In the preferred embodiment, alpha is valid only if the cap bit is setin the state register. The test bit is set when processing texturecomponents, in which case only red and green are valid.

The instruction set implementing decompression according to the presentinvention will now be described. FIG. 14A depicts the vertex instructionformat, an instruction that uses variable-length Huffman encoding torepresent a vertex. Position information is always present in thisinstruction.

(REP) The vertex replacement policy is as follows:

00--Restart clockwise

01--Restart counter-clockwise

10--Replace middle

11--Replace oldest

(M) mesh buffer push:

0--No push

1--Push

With reference to FIG. 14A, the position data consists of avariable-length Huffman tag (0 to 6 bits) followed by three data fieldsof equal length for the X, Y, and Z components, which are eitherΔ-values or absolute values. The data₋₋ len field for the entry in theposition Huffman table gives the length of each of the X, Y, and Zfields, the tag₋₋ len entry gives the length of the tag, and the abs₋₋rel entry tells whether the data is absolute data or is relative to theprevious vertex. The shift entry from the Huffman table gives thequantization level (number of trailing zeroes) for the data.

If the bnv bit is set in the state register, a normal is included. Theencoded normal has a Huffman tag followed by either two variable-lengthdata fields for Δu and Δv, or a fixed-length field for the sextant andoctant (6 bits) followed by two variable-length fields for u and v. Theformer encoding is for delta encodings of normals, while the latterencoding is for absolute encodings. The data₋₋ len, tag₋₋ len, abs₋₋rel, and shift fields from the normal Huffman table are used similarlyas entries from the position table.

FIG. 14B depicts vertex component data formats. If the bcv bit in thestate register is set, color is included with the vertex. The color isencoded similar the position, using three or four fields, but how thefields are used is determined by the tag table. If tagged absolute, thenx, y, z, r, g, b data is used. Absolute normals are used with sectantand octant fields. However, if the tag table indicates relative, deltanormals are used, and it sufficiences to send latitude and longitudedata (e.g., θ and φ, also referred to herein as u and v.

With further reference to FIG. 14B, a Huffman tag is followed by threeequal length fields for R, G, and B. The cap bit in the state registerindicates whether an additional field for α is included. The data₋₋ len,tag₋₋ len, abs₋₋ rel, and shift fields from the color Huffman table areused similarly as for entries from the position and normal tables.

The states of the vertex instruction set are as follows:

1. Latch next opcode; output X; shift barrel shift right unit 500 (seeFIG. 10) by ptag₋₋ len+pdata₋₋ len-pquant+2.

2. Merge; output Header.

3. Output Y; shift barrel shift right unit 500 (see FIG. 9) by pdata₋₋len-pquant.

4. Merge

5. Output Z; shift barrel shift right unit 500 (see FIG. 9) by pdata₋₋len-pquant.

6. Merge.

a. If (bnv)

i. if (absolute normal), goto 7,

ii. else goto 9./*relative normal*/

b. else If (rnt), goto 21,

c. else If (bcv) goto 13,

d. else If (rct) goto 22,

e. else Merge; branch to next instruction.

7. Latch next opcode; output sextant/octant; shift barrel shift rightunit 500 (see FIG. 9) by ntag₋₋ len+6.

8. Merge.

9. Output U.

a. If (absolute normal), shift barrel shift right unit 500 (see FIG. 9)by ndata₋₋ len-nquant.

b. else/*relative normal*/, latch next opcode; shift Bs2 by ntag₋₋len+ndata₋₋ len-nquant

10. Merge.

11. Output V.

12. Merge.

a. If (bcv), goto 13,

b. else If (rct), goto 22,

c. else Merge; branch to next instruction.

13. Latch next opcode; output R; shift barrel shift right unit 500 (seeFIG. 9) by ctag₋₋ len+cdata₋₋ len-cquant.

14. Merge

15. Output G; shift barrel shift right unit 500 (see FIG. 9) by cdata₋₋len-cquant.

16. Merge; if (tex), branch to next instruction.

17. Output B; shift barrel shift right unit 500 (see FIG. 9) by cdata₋₋len-cquant.

18. Merge; if (cap) branch to next instruction.

19. Output A; shift barrel shift right unit 500 (see FIG. 9) by cdata₋₋len-cquant.

20. Merge; branch to next instruction.

21. Output curr₋₋ normal.

a. If (bcv), goto 13,

b. else If (rct), goto 22,

c. else Merge; branch to next instruction.

22. Output curr₋₋ r.

23. Output curr₋₋ g. If (tex), Merge; branch to next instruction

24. Output curr₋₋ b. If (cap), Merge; branch to next instruction.

25. Output curr₋₋ a. Merge branch to next instruction.

FIG. 14C depicts the format for the set normal instruction. The setnormal instruction sets the value of the current normal registers. Thenormal data is encoded similarly as is normal data in the vertexinstruction, described herein. The states of the set normal instructionare as follows:

If (absolute normal)

1. Latch next opcode; output sextant/octant; shift barrel shift rightunit 500 (see FIG. 9) by ntag₋₋ len+8.

2. Merge.

3. Output U; shift barrel shift right unit 500 (see FIG. 9) by ndata₋₋len-nquant.

4. Merge.

5. Output V; shift barrel shift right unit 500 (see FIG. 9) by ndata₋₋len+nquant.

6. Merge; branch to next instruction. else/*relative normal*/

1. Latch next opcode; output dU; shift barrel shift right unit 500 (seeFIG. 9) by n₋₋ tag₋₋ len+ndata₋₋ len-nquant.

2. Merge.

3. Output dV; shift barrel shift right unit 500 (see FIG. 9) by ndata₋₋len-nquant.

4. Merge; branch to next instruction.

FIG. 14D depicts the set color instruction, an instruction that sets thevalue of the current color registers. Encoding of the color data issimilar to encoding of the color data in the vertex instruction. Thestates of the set color instruction are as follows:

1. Latch next opcode; output R; shift barrel shift right unit 500 (seeFIG. 9) by ctag₋₋ len+cdata₋₋ len-cquant+2.

2. Merge.

3. Output G; shift barrel shift right unit 500 (see FIG. 9) by cdata₋₋len-cquant.

4. Merge. If (tex), branch to next instruction.

5. Output B; shift barrel shift right unit 500 (see FIG. 9) by cdata₋₋len-cquant.

6. Merge. If (cap) branch to next instruction.

7. Output A; shift barrel shift right unit 500 (see FIG. 9) by cdata₋₋len-cquant.

8. Merge; branch to next instruction.

FIG. 14E is the preferred format for the mesh buffer referenceinstruction. This instruction causes data from an entry in the meshbuffer to be sent out to the format converter as the next vertex. Withreference to FIG. 14E, the index indicates the entry from the meshbuffer to send. The newest entry in the mesh buffer has index 0, and theoldest has index 15. REP, the above-described replacement policy for thevertex instruction, is the same as used for the mesh buffer referenceinstruction. The states for the mesh buffer reference instruction are asfollows:

1. Latch next opcode; output Header; shift barrel shift right unit 500(see FIG. 9) by 9.

2. Output X from mesh buffer.

3. Output Y from mesh buffer.

4. Output Z from mesh buffer.

a. If (bnv or rnt) goto 5,

b. else If (bcv or rct) goto 6,

c. else Merge; branch to next instruction.

5. If (bnv), output Normal from mesh buffer, else if (rnt) output curr₋₋normal.

a. If (bnv or rct) goto 6,

b. else Merge; branch to next instruction.

6. If (bcv), output R from mesh buffer, else if (rct) output curr₋₋ r.

7. If (bcv), output G from mesh buffer, else if (rct) output curr₋₋ g.If (tex), Merge; branch to next instruction.

8. If (bcv), output B from mesh buffer, else if (rct) output curr₋₋ b.If (cap), Merge; branch to next instruction.

9. If (bcv), output A from mesh buffer, else if (rct) output curr₋₋ a.Merge; branch to next instruction.

FIG. 14F depicts the set state instruction, which sets the bits thedecompression unit state register. The states for the set stateinstruction are as follows:

1. Latch next opcode; shift barrel shifter 2 by 11 bits.

2. Merge; branch instruction

3.

FIG. 14G depicts the set table instruction, which sets Huffman tableentries. The table selection is as follows:

00--Position table

01--Color table

10--Normal table

11--Undefined

The tag length is derived from the address. The nine bits in the entryfield correspond to the absolute/relative bit, data length, and shiftamount fields of the Huffman table entries. (The preferred format of theHuffman table entries has been described earlier herein.) The states ofthe set table instruction are as follows:

1. Latch next opcode; send address and entry to Huffman tables; shiftbarrel shift right unit 500 (see FIG. 9) by 23.

2. Merge; branch to next instruction.

Table 5 shows the preferred Huffman Table Fill Codes.

                  TABLE 5                                                         ______________________________________                                                 Entries               Fill                                           Address  Filled      Tag Length                                                                              Range                                          ______________________________________                                        0tttttt  1           6         tttttt                                         10ttttt  2           5         ttttt0-ttttt1                                  110tttt  4           4         tttt00-tttt11                                  1110ttt  8           3         ttt000-ttt111                                  11110tt  16          2         tt0000-tt1111                                  111110t  32          1         t00000-                                                                       t11111                                         1111110  64          0         Entire table                                   ______________________________________                                    

FIG. 14H depicts the passthrough instruction, which allows passthroughdata to be encoded in the compressed-data stream. The length of theinstruction preferably is 64-bits. Aligning successive passthroughinstructions to a 64-bit boundary allows for patching of passthroughdata in the encoded stream. The states for the passthrough instructionare as follows:

1. Latch next opcode; read address, shift barrel shift right unit 500(see FIG. 9) by 32 bits.

2. Merge.

3. Output data, shift barrel shift right unit 500 (see FIG. 9) by 32bits.

4. Merge; branch to next instruction.

FIG. 14I depicts the variable-length NOP ("VNOP") instruction, whichencodes a variable number of 0 bits in the data stream. The five-bitcount shown in FIG. 14I designates the number of 0 bits that follow.This instruction is implicitly used for the start of the data stream.This instruction may also be used to pad the data stream to 32-bit or64-bit boundaries, or encoding regions, for later patching. The statesfor this instruction are:

1 Latch next opcode; read count; barrel shift right unit 500 (see FIG.9) by 13 bits;

2. Merge.

3. Barrel shift right unit reads "count" positions;

4. Merge; branch to next instruction.

FIG. 14J shows the skip 8 instruction, whose states are:

1. Latch next opcode; shift barrel shift right unit 500 (see FIG. 9) by16 bits;

2. Merge; branch to next instruction.

It will be appreciated that it may be advantageous to reduce bandwidthrequirements between devices by not decompressing a data stream at asingle point in a decompression system. The present invention canprovide parallel decompression of a data stream by providing anadditional command advising the arrival of a given number of data wordsthat may be processed in parallel.

The present invention can recognize the presence of such parallelopportunities by the presence of mark bits, and cause the stated numberof data words to be shuttled to other processors within the system, forparallel decompression. Further, it is then permissible to jump aheadthe given number of words in the data stream to arrive at the next datathat is not eligible for parallel processing.

The present invention can also provide morphing capability to eliminateany abrupt perception gap in viewing a decompressed three-dimensionalobject. Within the decompressed data stream, it is possible to specifyvertices as linear or other interpolations of vertices that are actuallypresent or have previously been decompressed. Assume, for example, thatthe three-dimensional object is a tiger. At a far distance, no teeth arepresent in the tiger's mouth, yet at near distance teeth are present.The present invention can provide a seamless transition such that asdistance to the tiger shrinks, the teeth grow, with no sudden changeseen between a toothless tiger and a toothed tiger.

Modifications and variations may be made to the disclosed embodimentswithout departing from the subject and spirit of the invention asdefined by the following claims.

What is claimed is:
 1. A method for decompressing a plurality ofvariable-length instructions, said method comprising:receiving a datastream which includes a first instruction of said plurality ofvariable-length instructions, wherein said first instruction isexecutable to perform a first operation and includes one or moreheader-body pairs, wherein each of said one or more header-body pairsincludes a fixed-length header portion followed in said data stream by avariable-length body portion; decompressing a first header-body pairwhich includes a first header portion followed in said data stream by afirst body portion, wherein said first header portion and said firstbody portion include a first data portion having one or more data valuesusable for performing said first operation, wherein said decompressingsaid first header-body pair includes:accessing said first header portionfrom said data stream by selecting a predetermined number of bits,wherein said first header portion includes sufficient information todetermine a length value of said first body portion; determining saidlength value of said first body portion from said first header portion;accessing said first body portion from said data stream by selecting anumber of bits equal to said length value of said first body portion;utilizing said one or more data values in said first data portion inorder to perform said first operation.
 2. The method of claim 1, whereinsaid utilizing said one or more data values includes converting said oneor more data values in said first data portion to a format recognized bysaid first instruction.
 3. The method of claim 2, wherein said firstheader portion includes a first opcode value which is indicative ofinformation included in said first instruction.
 4. The method of claim3, wherein said first opcode value indicates that said first bodyportion is of a predetermined length, and wherein said determining saidlength value of said first body portion includes utilizing saidpredetermined length as said length value of said first body portion. 5.The method of claim 4, wherein a length of said first opcode value isless than a length of said first header portion.
 6. The method of claim5, wherein said decompressing said first header-body pair includesaccessing a first part of said first data portion which is included in aremaining number of bits of said first header portion which follow saidfirst opcode value.
 7. The method of claim 6, wherein a length of saidfirst data portion is greater than said remaining number of bits of saidfirst header portion which follow said first opcode value, and whereinsaid decompressing said first header-body pair further includesaccessing a remaining part of said first data portion from said firstbody portion.
 8. The method of claim 6, wherein a length of said firstdata portion is less than or equal to said remaining number of bits ofsaid first header portion which follow said first opcode value, andwherein said first part of said first data portion includes all of saidfirst data portion.
 9. The method of claim 8, wherein said first bodyportion is of zero length.
 10. The method of claim 4, wherein a lengthof said first opcode value is equal to a length of said first headerportion, and wherein said first data portion is entirely included insaid first body portion.
 11. The method of claim 3, wherein said firstopcode value indicates that said first body portion is of a variablelength determinable by information in a second part of said first headerportion which follows said first opcode value, wherein said second partof said first header portion includes a variable-length first tag value.12. The method of claim 11, wherein a combined length of said firstopcode value and said first tag value is less than a total number ofbits in said first header portion.
 13. The method of claim 12, whereinsaid decompressing said first header-body portion includes accessing afirst part of said first data portion which is included in a remainingnumber of bits in said first header portion which follow said firstopcode value and said first tag value.
 14. The method of claim 13,wherein a length of said first data portion is greater than saidremaining number of bits in said first header portion which follow saidfirst opcode value and said first tag value, and wherein saiddecompressing said first header-body pair further includes accessing aremaining part of said first data portion from said first body portion.15. The method of claim 13, wherein a length of said first data portionis less than or equal to said remaining number of bits of said firstheader portion which follow said first opcode value and said first tagvalue, and wherein said first part of said first data portion includesall of said first data portion.
 16. The method of claim 15, wherein saidfirst body portion is of zero length.
 17. The method of claim 11,wherein a combined length of said first opcode value and said first tagvalue is equal to a total number of bits in said first header portion,and wherein said first data portion is entirely included in said firstbody portion.
 18. The method of claim 12, wherein said first headerportion is of a predetermined length and includes a variable-lengthfirst tag value, and wherein said length value of said first bodyportion is determinable from said first tag value in said first headerportion.
 19. The method of claim 18, wherein a length of said first tagvalue is less than said predetermined length of said first headerportion.
 20. The method of claim 19, wherein said decompressing saidfirst header-body pair includes accessing a first part of said firstdata portion from a remaining number of bits in said first headerportion which follow said first tag value.
 21. The method of claim 20,wherein a length of said first data portion is greater than saidremaining number of bits in said first header portion which follow saidfirst tag value, and wherein said decompressing said first header-bodypair further includes accessing a remaining part of said first dataportion from said first body portion.
 22. The method of claim 20,wherein a length of said first data portion is less than or equal tosaid remaining number of bits in said first header portion which followsaid first tag value, and wherein said first part of said first dataportion includes all of said first data portion.
 23. The method of claim22, wherein said first body portion is of zero length.
 24. The method ofclaim 18, wherein a length of said first tag value is equal to a totalnumber of bits in said first header portion, and wherein said first dataportion is entirely included in said first body portion.
 25. The methodof claim 11, wherein said second part of said first header portion isusable to determine a length value of said first tag value and saidlength value of said first body portion.
 26. The method of claim 25,further comprising using said second part of said first header portionto select a first set of decompression parameters, and still furthercomprising using said first set of decompression parameters to performsaid converting said one or more data values in said first data portion.27. The method of claim 26, wherein said first set of decompressionparameters are selected from the group consisting of (i) an indicationof whether said one or more data values in said first data portion areabsolute or relative values, and (ii) a data normalization coefficientusable for scaling said one or more data values to a given numericrange.
 28. The method of claim 11, wherein said second part of saidfirst header portion is of a predetermined length.
 29. The method ofclaim 28, further comprising presenting said second part of said firstheader portion to a decompression table, wherein said decompressiontable stores a plurality of sets of decompression parameters usable fordecompressing various data portions in said plurality of variable-lengthinstructions.
 30. The method of claim 29, wherein said decompressiontable includes a first set of decompression parameters which correspondto said first data portion, and wherein said decompression tableincludes a number of duplicate entries which each include said first setof decompression parameters, and wherein said number of duplicateentries is equal to 2^(x), wherein x is equal to a number of bits insaid predetermined length of said second part of said header portionminus a bit length of said first tag value, and wherein saiddecompression table is configured to output a first entry from saidnumber of duplicate entries in response to receiving said second part ofsaid first header portion.
 31. The method of claim 30, wherein saidfirst entry includes said length value of said first tag value and saidlength value of said first body portion.
 32. The method of claim 31,wherein said decompressing said first header-body pair includes usingsaid length value of said first tag value to determine a remainingnumber of bits in said second part of said first header portion whichare after said first tag value.
 33. The method of claim 32, wherein saidlength of said first tag value and said length value of said first bodyportion are usable to access said first data portion.
 34. The method ofclaim 32, wherein said first entry further includes decompressionparameters selected from the group consisting of (i) an indication ofwhether said one or more data values in said first data portion areabsolute or relative values, and (ii) a data normalization coefficientusable for scaling said one or more data values to a given numericrange.
 35. The method of claim 18, wherein said first header portion isusable to determine a length value of said first tag value and saidlength value of said first body portion.
 36. The method of claim 35,further comprising using said first header portion to select a first setof decompression parameters, and still further comprising using saidfirst set of decompression parameters to perform said converting saidone or more data values in said first data portion.
 37. The method ofclaim 36, wherein said first set of decompression parameters areselected from the group consisting of (i) an indication of whether saidone or more data values in said first data portion are absolute orrelative values, and (ii) a data normalization coefficient usable forscaling said one or more data values to a given numeric range.
 38. Themethod of claim 18, wherein said first header portion is of apredetermined length.
 39. The method of claim 38, further comprisingpresenting said first header portion to a decompression table, whereinsaid decompression table stores a plurality of sets of decompressionparameters usable for decompressing various portions of data in saidplurality of variable-length instructions.
 40. The method of claim 39,wherein said decompression table includes a first set of decompressionparameters which correspond to said first data portion, and wherein saiddecompression table includes a number of duplicate entries which eachinclude said first set of decompression parameters, and wherein saidnumber of duplicate entries is equal to 2^(x), wherein x is equal to anumber of bits in said predetermined length of said first header portionminus a bit length of said first tag value, and wherein saiddecompression table is configured to output a first entry in response toreceiving said first header portion.
 41. The method of claim 40, whereinsaid first entry includes said length value of said first tag value andsaid length value of said first body portion.
 42. The method of claim40, further comprising using said length value of said first tag valueto determine a remaining number of bits in said first header portionfollowing said first tag value.
 43. The method of claim 42, wherein saidlength of said first tag value and said length value of said first bodyportion are usable to access said first data portion.
 44. The method ofclaim 42, wherein said first entry further includes decompressionparameters selected from the group consisting of (i) an indication ofwhether said of one or more data values in said first data portion areabsolute or relative values, and (ii) a data normalization coefficientusable for scaling said one or more data values to a given numericrange.
 45. The method of claim 1, wherein said plurality ofvariable-length instructions further includes a second header-body pairhaving a second fixed-length header portion followed in said data streamby a second variable-length body portion, wherein said second headerportion includes information sufficient to determine a length value ofsaid second body portion, and wherein said second header portion andsaid second body portion include a second data portion.
 46. The methodof claim 45, wherein said plurality of variable-length instructionsfurther includes a third header-body pair having a third fixed-lengthheader portion followed in said data stream by a third variable-lengthbody portion, wherein said third header portion includes informationsufficient to determine a length value of said third body portion, andwherein said third header portion and said third body portion include athird data portion.
 47. The method of claim 46, wherein said firstheader portion is separated from said first body portion in said datastream by said second body portion and said third header portion. 48.The method of claim 47, wherein said second header portion is locatedprior to said first header portion in said data stream, and wherein saidthird body portion is located subsequent to said first body portion insaid data stream.
 49. The method of claim 48, wherein said second headerportion and said second body portion are included in a secondinstruction of said plurality of variable-length instructions, andwherein said third header portion and said third body portion areincluded in said first instruction.
 50. The method of claim 48, whereinsaid second header portion and said second body portion are included ina second instruction of said plurality of variable-length instructions,and wherein said third header portion and said third body portion areincluded in a third instruction of said plurality of variable-lengthinstructions.
 51. The method of claim 48, wherein said second headerportion and said second body portion are included in said firstinstruction, and wherein said third header portion and said third bodyportion are included in said first instruction.
 52. The method of claim48, wherein said second header portion and said second body portion areincluded in said first instruction, and wherein said third headerportion and said third body portion are included in a second instructionof said plurality of variable-length instructions.
 53. The method ofclaim 49, wherein said plurality of variable-length instructionsincludes a fourth header-body pair as part of a third instruction,wherein said fourth header-body pair includes a fourth fixed-lengthheader portion followed in said data stream by a fourth variable-lengthbody portion, wherein said fourth header portion includes informationsufficient to determine a length value of said fourth body portion, andwherein said fourth header portion and said fourth body portion includea fourth data portion.
 54. The method of claim 53, wherein said fourthheader portion is located subsequent to said first body portion andprior to said third body portion in said data stream, and wherein saidfourth body portion is located subsequent to said third body portion insaid data stream.
 55. The method of claim 27, wherein said firstinstruction further includes a fixed-length second header portion and avariable-length second body portion, and wherein said second headerportion includes a second tag value which is usable to determine alength value of said second body portion.
 56. The method of claim 55,wherein said second header portion is located subsequent to said firstheader portion and prior to said first body portion in said data stream,and wherein said second body portion is located subsequent to said firstbody portion in said data stream.
 57. The method of claim 55, furthercomprising determining said length value of said second body portion byproviding said second tag value to a second decompression table, whereinsaid second decompression table is configured to provide a second set ofdecompression parameters in response to receiving said second tag value.58. The method of claim 57, further comprising decompressing informationin said second body portion by utilizing said second set ofdecompression parameters provided from said second decompression tablein response to said second tag value.
 59. The method of claim 58,wherein said second set of decompression parameters are selected fromthe group consisting of (i) length of said second tag value, (ii) saidlength value of said second body portion, (iii) an indication of whethersaid second body portion includes an absolute or relative value, and(iv) a data normalization coefficient for one or more data values insaid second body portion.
 60. The method of claim 4, wherein saidplurality of variable-length instructions represent compressed 3-Dgeometry data, and wherein said plurality of variable-lengthinstructions include information corresponding to plurality of vertexparameter values.
 61. The method of claim 60, wherein said firstinstruction includes an indication of whether to set a color bundlingbit and a normal bundling bit.
 62. The method of claim 61, furthercomprising setting said color bundling bit in response to said firstinstruction including an indication to set said color bundling bit,wherein said color bundling bit being set indicates that variable-lengthinstructions following said first instruction which specify vertexparameter values include one or more color values in addition to one ormore position values.
 63. The method of claim 61, further comprisingsetting said normal bundling bit in response to said first instructionincluding an indication to set said normal bundling bit, wherein saidnormal bundling bit being set indicates that variable-lengthinstructions following said first instruction which specify vertexparameter values include one or more normal values in addition to one ormore position values.
 64. The method of claim 60, wherein said firstopcode value includes an indication to perform a reference to a meshbuffer, wherein said mesh buffer includes vertex parameter valuesspecified by prior instructions.
 65. The method of claim 64, furthercomprising performing said reference to said mesh buffer, wherein saidreference to said mesh buffer is usable to specify one or more vertexparameter values corresponding to one of a plurality of vertices in saidcompressed 3-D geometry data.
 66. The method of claim 60, wherein saidfirst opcode value includes an indication that said first instruction isexecutable to set one or more decompression table entries.
 67. Themethod of claim 66, further comprising setting said one or moredecompression table entries prior to said accessing said first headerportion, wherein said one or more decompression table entries are usableto decompress body portions of instructions subsequent to said firstinstruction.
 68. The method of claim 27, wherein said plurality ofvariable-length instructions represent compressed 3-D geometry data, andwherein said plurality of variable-length instructions includeinformation corresponding to a plurality of vertex parameter values. 69.The method of claim 68, wherein said first opcode value indicates thatsaid first instruction specifies vertex parameters corresponding to aportion of said compressed 3-D geometry data.
 70. The method of claim69, wherein said first body portion includes vertex positioninformation.
 71. The method of claim 70, wherein said vertex positioninformation is absolutely specified.
 72. The method of claim 70, whereinsaid vertex position information is delta-encoded with respect topreviously specified vertex position information.
 73. The method ofclaim 70, wherein said first instruction further includes a fixed-lengthsecond header portion followed by a variable-length second body portion,and wherein said second header portion includes information sufficientto determine a length value of said second body portion.
 74. The methodof claim 70, further comprising:accessing a second header portion fromsaid data stream by selecting said predetermined number of bits, whereinsaid second header portion includes sufficient information to determinea length value of a second body portion which follows said second headerportion in said data stream, wherein said second header portion and saidsecond body portion collectively include a second data portion;determining said length value of said second body portion from saidsecond header portion; accessing said second body portion from said datastream by selecting a number of bits equal to said length value of saidsecond body portion; decompressing information in said second dataportion using information in said second header portion.
 75. The methodof claim 74, wherein said determining said length value of said secondbody portion includes providing said second tag value to a seconddecompression table, wherein said second decompression table isconfigured to provide a second set of decompression parameters inresponse to receiving said second tag value.
 76. The method of claim 75,wherein said decompressing information in said second body portionincludes utilizing said second set of decompression parameters providedfrom said second decompression table in response to said second tagvalue.
 77. The method of claim 76, wherein said second set ofdecompression parameters are selected from the group consisting of (i)length of said second tag value, (ii) said length value of said secondbody portion, (iii) an indication of whether said second body portionincludes an absolute or relative value, and (iv) a data normalizationcoefficient for one or more data values in said second body portion. 78.The method of claim 77, wherein said second body portion includes vertexcolor information, and wherein said second decompression table is acolor value decompression table.
 79. The method of claim 78, whereinsaid second body portion includes vertex color information in responseto a bundle colors with vertices bit being set in response to apreviously specified instruction in said data stream.
 80. The method ofclaim 78, wherein said vertex color information is absolutely specified.81. The method of claim 68, wherein said vertex color information isdelta-encoded with respect to previously specified vertex colorinformation.
 82. The method of claim 76, wherein said second bodyportion includes vertex normal information, and wherein said seconddecompression table is a normal value decompression table.
 83. Themethod of claim 82, wherein said second body portion includes vertexnormal information in response to a bundle normals with vertices bitbeing set in response to a previously specified instruction in said datastream.
 84. The method of claim 82, wherein said vertex normalinformation is absolutely specified.
 85. The method of claim 82, whereinsaid vertex normal information is delta-encoded with respect topreviously specified vertex normal information.
 86. The method of claim73, wherein said second header portion is located subsequent to saidfirst header portion and prior to said first body portion in said datastream, and wherein said second body portion is located subsequent tosaid first body portion in said data stream.
 87. A method fordecompressing a plurality of variable-length instructions, said methodcomprising:receiving a data stream which includes said plurality ofvariable-length instructions; accessing a fixed-length first headerportion from said data stream by selecting a predetermined number ofbits, wherein said first header portion includes sufficient informationto determine a length value of a first body portion which is subsequentto said first header portion in said data stream; accessing avariable-length second body portion from said data stream whichcorresponds to a second header portion previously specified in said datastream, wherein said second body portion immediately follows said firstheader portion in said data stream; determining said length value ofsaid first body portion from said first header portion; accessing afixed-length third header portion from said data stream by selectingsaid predetermined number of bits, wherein said third header portionincludes sufficient information to determine a length value of a thirdbody portion which is subsequent to said first body portion in said datastream accessing said variable-length first body portion from said datastream by selecting a number of bits equal to said length value of saidfirst body portion; decompressing a first data portion in accordancewith information in said first header portion, wherein said first dataportion is included in said first header portion and said first bodyportion.
 88. The method of claim 87, wherein said first header portionand said first body portion are included in a first instruction of saidplurality of variable-length instructions.
 89. The method of claim 88,wherein said second header portion and said second body portion areincluded in a second instruction of said plurality of variable-lengthinstructions, and wherein said third header portion and said third bodyportion are included in said first instruction.
 90. The method of claim88, wherein said second header portion and said second body portion areincluded in a second instruction of said plurality of variable-lengthinstructions, and wherein said third header portion and said third bodyportion are included in a third instruction of said plurality ofvariable-length instructions.
 91. The method of claim 88, wherein saidsecond header portion, said second body portion, said third headerportion, and said third body portion are included in said firstinstruction.
 92. The method of claim 88, wherein said second headerportion and said second body portion are included in said firstinstruction, and wherein said third header portion and said third bodyportion are included in a second instruction of said plurality ofvariable-length instructions.
 93. A method for decompressing compressed3-D geometry data, wherein said compressed 3-D geometry data includes aplurality of variable-length instructions which include informationdescribing a plurality of vertex parameter values, comprising:receivinga data stream which includes a first instruction of said plurality ofvariable-length instructions, wherein said first instruction includes afixed-length first header portion followed in said data stream by avariable-length first body portion, wherein said first header portionand said first body portion include a first data portion; accessing saidfirst header portion from said data stream by selecting a predeterminednumber of bits, wherein said first header portion includes sufficientinformation to determine a length value of said first body portion;determining said length value of said first body portion from said firstheader portion; accessing said first body portion from said data streamby selecting a number of bits equal to said length value of said firstbody portion; decompressing one or more data values in said first dataportion utilizing decompression parameters obtained using information insaid first header portion.
 94. The method of claim 93, wherein saidfirst header portion includes a first opcode value which indicates thatsaid first instruction includes vertex parameter values corresponding toa first vertex of a plurality of vertices in said compressed 3-Dgeometry data.
 95. The method of claim 94, wherein said first bodyportion includes one or more position values of said first vertex. 96.The method of claim 94, wherein said first header portion includes afirst tag value usable to determine said length value of said first bodyportion, and wherein said determining said length value of said firstbody portion includes providing said first tag value to a firstdecompression table, wherein said first decompression table isconfigured to provide a first set of decompression parameters inresponse to receiving said first tag value.
 97. The method of claim 96,wherein said first set of decompression parameters are selected from thegroup consisting of (i) length of said first tag value, (ii) said lengthvalue of said first body portion, (iii) an indication of whether saidone or more position values are absolute or relative values, and (iv) adata normalization coefficient for said one or more position values. 98.The method of claim 96, wherein said first set of decompressionparameters includes an indication that said one or more data values insaid first body portion are absolutely specified.
 99. The method ofclaim 96, wherein said decompressing one or more data values in saidfirst body portion includes shifting said one or more position values insaid first body portion by an amount specified by said datanormalization coefficient in order to generate decompressed one or moreposition values represented within a specified numeric range.
 100. Themethod of claim 96, wherein said first set of decompression parametersincludes an indication that said one or more position values in saidfirst body portion are specified relative to previously specifiedposition values.
 101. The method of claim 100, wherein saiddecompressing one or more data values in said first body portionincludes shifting said one or more position values in said first bodyportion by an amount specified by said data normalization coefficient,thereby generating one or more position values represented within aspecified numeric range.
 102. The method of claim 101, wherein saiddecompressing one or more data values in said first body portion furtherincludes adding said one or more position values represented within saidfirst numeric range to said previously specified position values,thereby generating final decompressed position values.
 103. The methodof claim 95, wherein said first instruction further includes afixed-length second header portion followed in said data stream by avariable-length second body portion, wherein said second header portionand said second body portion include a second data portion.
 104. Themethod of claim 103, further comprising:accessing said second headerportion from said data stream by selecting said predetermined number ofbits, wherein said second header portion includes sufficient informationto determine a length value of said second body portion; determiningsaid length value of said second body portion from said second headerportion; accessing said second body portion from said data stream byselecting a number of bits equal to said length value of said secondbody portion; decompressing one or more data values in said second dataportion utilizing decompression parameters obtained using information insaid first header portion.
 105. The method of claim 104, wherein saidsecond body portion includes vertex parameters values selected from thegroup consisting of: (i) one or more color values of said first vertex,(ii) one or more normal values of said first vertex (iii) texture mapcoordinates of said first vertex, and (iv) surface material propertiesof said first vertex.
 106. The method of claim 104, wherein said secondheader portion includes a second tag value usable to determine saidlength value of said second body portion, and wherein said determiningsaid length value of said second body portion includes providing saidsecond tag value to a second decompression table, wherein said seconddecompression table is configured to provide a second set ofdecompression parameters in response to receiving said second tag value.107. The method of claim 106, wherein said second set of decompressionparameters are selected from the group consisting of (i) length of saidsecond tag value, (ii) said length value of said second body portion,(iii) an indication of whether said one or more data values in saidsecond body portion are absolute or relative values, and (iv) a datanormalization coefficient for said one or more data values in saidsecond body portion.
 108. The method of claim 107, wherein said secondset of decompression parameters includes an indication that said one ormore data values in said second body portion are absolutely specified.109. The method of claim 108, wherein said decompressing one or moredata values in said second body portion includes shifting said one ormore data values in said first body portion by an amount specified bysaid data normalization coefficient in order to generate decompressedone or more data values represented within a first numeric range. 110.The method of claim 107, wherein said second set of decompressionparameters includes an indication that said one or more data values insaid second body portion are specified relative previously specifieddata values.
 111. The method of claim 110, wherein said decompressingone or more data values in said second body portion includes shiftingsaid one or more data values in said second body portion by an amountspecified by said data normalization coefficient, thereby generating oneor more data values represented within a specified numeric range. 112.The method of claim 111, wherein said decompressing one or more datavalues in said second body portion further includes adding said one ormore data values represented within a specified numeric range to saidpreviously specified data values, thereby generating final decompresseddata values.
 113. The method of claim 103, wherein said second headerportion is subsequent to said first header portion and prior to saidfirst body portion in said data stream, and wherein said second bodyportion is subsequent to said first body portion in said data stream.114. A computer system for decompressing a plurality of variable-lengthinstructions within a data stream, comprising:an input unit coupled toreceive said data stream, wherein said data stream includes a firstheader-body pair having a fixed-length first header portion followed insaid data stream by a variable-length first body portion, wherein saidfirst header portion and said first body portion include a first dataportion having one or more data values; a control unit coupled to saidinput unit, wherein said control unit is configured to access said firstheader portion from said data stream by selecting a predetermined numberof bits, and wherein said control unit is configured to utilizeinformation in said first header portion in order to determine a firstset of decompression parameters, wherein said first set of decompressionparameters includes a length value of said first body portion, andwherein said control unit is configured to access said first bodyportion from said data stream by selecting a number of bits equal tosaid length value of said first body portion; a decompression unitcoupled to said input unit and said control unit, and wherein saiddecompression unit is coupled to receive said first set of decompressionparameters and said first data portion, and wherein said decompressionunit is configured to decompress said one or more data values in saidfirst data portion according to said first set of decompressionparameters.
 115. The computer system of claim 114, wherein said firstheader portion includes an first opcode value which indicates that saidfirst body portion is of a predetermined length.
 116. The computersystem of claim 115, wherein said control unit is configured to utilizesaid predetermined length as said length value of said first bodyportion.
 117. The computer system of claim 116, wherein a length of saidfirst opcode value is equal to said predetermined number of bits in saidfirst header portion, and wherein said control unit is configured toconvey said first data portion to said decompression unit entirely fromsaid first body portion.
 118. The computer system of claim 116, whereina length of said first opcode value is less than said predeterminednumber of bits in said first header portion, and wherein said opcodevalue occupies a first part of said first header portion.
 119. Thecomputer system of claim 118, where in a second part of said firstheader portion includes bits which follow said first part of said firstheader portion.
 120. The computer system of claim 119, wherein saidfirst header portion includes an first opcode value which indicates thatsaid first body portion is of a length determinable using information insaid second part of said first header portion.
 121. The computer systemof claim 120, further comprising a first decompression table coupled tosaid control unit, wherein said first decompression table includes oneor more sets of decompression parameters usable for decompressing datavalues compressed within said data stream.
 122. The computer system ofclaim 121, wherein said control unit is configured to convey said secondpart of said first header portion to said first decompression table, andwherein said first decompression table is configured to convey a firstset of decompression parameters to said control unit in responsethereto, wherein said second part of said first header portion forms anindex in to said first decompression table.
 123. The computer system ofclaim 122, wherein said second part of said first header portionincludes a variable-length first tag value.
 124. The computer system ofclaim 123, wherein said first decompression table includes a number ofduplicate entries each storing said first set of decompressionparameters, wherein said number of duplicate entries is dependent uponthe length of said first tag value relative to a number of bits used toindex into said first decompression table.
 125. The computer system ofclaim 123, wherein said first set of decompression parameters includes alength of said first tag value and a length of said first body portion.126. The computer system of claim 125, wherein said length of said firsttag value is equal to a length of said second part of said first headerportion, and wherein said control unit is configured to convey saidfirst data portion to said decompression unit entirely from said firstbody portion.
 127. The computer system of claim 126, wherein said lengthof said first tag value is less than a length of said second part ofsaid first header portion.
 128. The computer system of claim 127,wherein a length of said first data portion is less than or equal to aremaining number of bits in said second part of said first headerportion which follow said first tag value, and wherein said control unitis configured to convey said first data portion to said decompressionunit entirely from said remaining number of bits in said second part ofsaid first header portion.
 129. The computer system of claim 127,wherein a length of said first data portion is greater than a remainingnumber of bits in said second part of said first header portion whichfollow said first tag value, and wherein said control unit is configuredto convey a beginning part of said first data portion to saiddecompression unit from said remaining number of bits in said secondpart of said first header portion, and wherein said control unit isfurther configured to convey a remaining part of said first data portionto said decompression unit from said first body portion.
 130. Thecomputer system of claim 114, wherein said first header-body pair arepart of a first variable-length instruction, wherein said firstvariable-length instruction includes a previous header-body pair havingan first opcode value which indicates that said first body portion is ofa length determinable using information in said first header portion.131. The computer system of claim 130, further comprising a firstdecompression table coupled to said control unit, wherein said firstdecompression table includes one or more sets of decompressionparameters usable for decompressing data values compressed within saiddata stream.
 132. The computer system of claim 131, wherein said controlunit is configured to convey said first header portion to said firstdecompression table, and wherein said first decompression table isconfigured to convey a first set of decompression parameters to saidcontrol unit in response thereto, wherein bits of said first headerportion form an index into said first decompression table.
 133. Thecomputer system of claim 132, wherein said first header portion includesa variable-length first tag value, wherein said first tag value islocated in a first part of said first header portion.
 134. The computersystem of claim 133, wherein said first decompression table includes anumber of duplicate entries each storing said first set of decompressionparameters, wherein said number of duplicate entries is dependent uponthe length of said first tag value relative to a number of bits used toindex into said first decompression table.
 135. The computer system ofclaim 133, wherein said first set of decompression parameters includes alength of said first tag value and a length of said first body portion.136. The computer system of claim 135, wherein said length of said firsttag value is equal to a length of said first header portion, and whereinsaid control unit is configured to convey said first data portion tosaid decompression unit entirely from said first body portion.
 137. Thecomputer system of claim 135, wherein said length of said first tagvalue is less than a length of said first header portion.
 138. Thecomputer system of claim 137, wherein a length of said first dataportion is less than or equal to a remaining number of bits in saidfirst header portion which follow said first tag value, and wherein saidcontrol unit is configured to convey said first data portion to saiddecompression unit entirely from said remaining number of bits in saidfirst header portion which follow said first tag value.
 139. Thecomputer system of claim 137, wherein a length of said first dataportion is greater than a remaining number of bits in said first headerportion which follow said first tag value, and wherein said control unitis configured to convey a beginning part of said first data portion tosaid decompression unit from said remaining number of bits in said firstheader portion which follow said first tag value, and wherein saidcontrol unit is further configured to convey a remaining part of saidfirst data portion to said decompression unit from said first bodyportion.
 140. The computer system of claim 114, wherein said data streamfurther includes a second header-body pair having a fixed-length secondheader portion followed in said data stream by a variable-length secondbody portion, wherein said second header portion and said second bodyportion include a second data portion having one or more data values.141. The computer system of claim 140, wherein said second headerportion is subsequent to said first header portion and prior to saidfirst body portion in said data stream, and wherein said second bodyportion is located subsequent to said first body portion in said datastream.
 142. The computer system of claim 141, wherein said data streamfurther includes a previous header-body pair having a fixed-lengthprevious header portion followed in said data stream by avariable-length previous body portion, wherein said previous headerportion and said previous body portion include a previous data portionhaving one or more data values.
 143. The computer system of claim 142,wherein said previous header portion is located prior to said firstheader portion in said data stream, and wherein said previous bodyportion is located subsequent to said first header portion and prior tosaid second header portion in said data stream.
 144. The computer systemof claim 143, wherein said data stream further includes a thirdheader-body pair having a fixed-length third header portion followed insaid data stream by a variable-length third body portion, wherein saidthird header portion and said third body portion include a third dataportion having one or more data values.
 145. The computer system ofclaim 144, wherein said third header portion is located subsequent tosaid first body portion and prior to said second body portion in saiddata stream, and wherein said third body portion is located subsequentto said second body portion in said data stream.
 146. The computersystem of claim 145, wherein said control unit is configured to accesssaid previous header portion prior to accessing said first headerportion.
 147. The computer system of claim 146, wherein said controlunit is configured to determine a length value of said previous bodyportion concurrently with accessing said first header portion.
 148. Thecomputer system of claim 147, wherein said control unit is configured toaccess said previous body portion from said data stream by selecting anumber of bits from said data stream equal to said length value of saidprevious body portion, wherein said control unit is configured to accesssaid previous body portion after accessing said first header portion anddetermining said length value of said previous body portion.
 149. Thecomputer system of claim 148, wherein said control unit is configured toaccess said second header portion from said data stream after accessingsaid previous body portion, wherein said control unit is configured toaccess said second header portion by selecting said predetermined numberof bits from said data stream.
 150. The computer system of claim 149,wherein said control unit is configured to determine said length valueof said first body portion during accessing of said previous bodyportion and said second header portion.
 151. The computer system ofclaim 150, wherein accessing of said first body portion from said datastream by said control unit is configured to occur after accessing saidsecond header portion and determining said length value of said firstbody portion.
 152. The computer system of claim 151, wherein saidcontrol unit is configured to access said third header portion from saiddata stream after accessing said first body portion, wherein saidcontrol unit is configured to access said third header portion byselecting said predetermined number of bits from said data stream. 153.The computer system of claim 152, wherein said control unit isconfigured to determine said length value of said second body portionduring accessing of said first body portion and said third headerportion.
 154. The computer system of claim 153, wherein accessing ofsaid second body portion from said data stream by said control unit isconfigured to occur after accessing said third header portion anddetermining said length value of said second body portion.
 155. Thecomputer system of claim 154, wherein said control unit is configured todetermine said length value of said third body portion during accessingof said second body portion.
 156. The computer system of claim 155,wherein accessing of said third body portion from said data stream bysaid control unit is configured to occur after accessing said secondbody portion and determining said length value of said third bodyportion.
 157. The computer system of claim 156, wherein said firstheader portion and said first body portion are included in a firstinstruction of said plurality of variable-length instructions.
 158. Thecomputer system of claim 157, wherein said second header portion andsaid second body portion are included in said first instruction, andwherein said third header portion and said third body portion areincluded in said first instruction.
 159. The computer system of claim157, wherein said second header portion and said second body portion areincluded in a second instruction of said plurality of variable-lengthinstructions, and wherein said third header portion and said third bodyportion are included in a third instruction of said plurality ofvariable-length instructions.
 160. The computer system of claim 157,wherein said second header portion and said second body portion areincluded in a second instruction of said plurality of variable-lengthinstructions, and wherein said third header portion and said third bodyportion are included in said second instruction.
 161. The computersystem of claim 157, wherein said second header portion and said secondbody portion are included in said first instruction, and wherein saidthird header portion and said third body portion are included in asecond instruction of said plurality of variable-length instructions.162. The computer system of claim 161, wherein said previous headerportion and said previous body portion are included in a previousinstruction of said plurality of variable-length instructions.
 163. Thecomputer system of claim 114, wherein said plurality of variable-lengthinstructions represent compressed 3-D geometry data, and wherein saidplurality of variable-length instructions include informationcorresponding to a plurality of vertex parameter values.
 164. Thecomputer system of claim 163, wherein said first header portion includesa first tag value usable for determining said first set of decompressionparameters.
 165. The computer system of claim 164, wherein said firstset of decompression parameters are selected from the group consistingof (i) length of said first tag value, (ii) said length value of saidfirst body portion, (iii) an indication of whether said first dataportion includes an absolute or relative value, and (iv) a first datanormalization coefficient for one or more data values in said first dataportion.
 166. The computer system of claim 165, wherein said first setof decompression parameters includes an indication that said one or moredata values in said first data portion are absolutely specified. 167.The computer system of claim 166, wherein said decompression unit isconfigured to shift said one or more data values in said first dataportion by an amount specified by said first data normalizationcoefficient in order to generate final decompressed values for saidfirst body portion.
 168. The computer system of claim 165, wherein saidfirst set of decompression parameters includes an indication that saidone or more data values in said first data portion are delta-encodedrelative to previously specified data values.
 169. The computer systemof claim 168, wherein said decompression unit is configured to shiftsaid one or more data values in said first data portion by an amountspecified by said first data normalization coefficient in order togenerate intermediate decompressed values for said first body portion.170. The computer system of claim 169, wherein said decompression unitis further configured to add said intermediate decompressed values forsaid first data portion to said previously specified data values inorder to generate final decompressed data values for said first dataportion.
 171. The computer system of claim 114, wherein said one or morevalues in said first data portion are of a first data type.
 172. Thecomputer system of claim 171, further comprising a first decompressiontable, wherein said first decompression table includes one or more setsof decompression parameters usable for decompressing data values of saidfirst data type compressed within said data stream, wherein said firstdecompression table is configured to convey said first set ofdecompression parameters to said control unit in response to receivinginformation in said first header portion.
 173. The computer system ofclaim 172, wherein said data stream includes a fixed-length secondheader portion followed in said data stream by a variable-length secondbody portion, wherein said second header portion and said second bodyportion include a second data portion having one or more data values ofa second data type.
 174. The computer system of claim 173, wherein saidcontrol unit is configured to access said second header portion fromsaid data stream by selecting a predetermined number of bits.
 175. Thecomputer system of claim 174, further comprising a second decompressiontable, wherein said second decompression table includes one or more setsof decompression parameters usable for decompressing data values of saidsecond data type compressed within said data stream, wherein said seconddecompression table is configured to convey a second set ofdecompression parameters to said control unit in response to receivinginformation from said second header portion conveyed by said controlunit.
 176. The computer system of claim 175, wherein said second set ofdecompression parameters includes a length value of said second bodyportion, and wherein said control unit is configured to access saidsecond body portion from said data stream by selecting a number of bitsequal to said length value of said second body portion.
 177. Thecomputer system of claim 176, wherein said second header portionincludes a second tag value, and wherein said second tag value forms aunique portion of an index into said second decompression table. 178.The computer system of claim 177, wherein said second set ofdecompression parameters are selected from the group consisting of (i)length of said second tag value, (ii) said length value of said secondbody portion, (iii) an indication of whether said second data portionincludes an absolute or relative value, and (iv) a second datanormalization coefficient for one or more data values in said seconddata portion.
 179. The computer system of claim 178, wherein said secondset of decompression parameters includes an indication that said one ormore data values in said second body portion are absolutely specified.180. The computer system of claim 179, wherein said decompression unitis configured to shift said one or more data values in said second dataportion by an amount specified by said second data normalizationcoefficient in order to generate final decompressed values for saidsecond data portion.
 181. The computer system of claim 178, wherein saidsecond set of decompression parameters includes an indication that saidone or more data values in said second data portion are delta-encodedrelative to previously specified data values.
 182. The computer systemof claim 181, wherein said decompression unit is configured to shiftsaid one or more data values in said second data portion by an amountspecified by said second data normalization coefficient in order togenerate intermediate decompressed values for said first data portion.183. The computer system of claim 182, wherein said decompression unitis further configured to add said intermediate decompressed values forsaid second data portion to said previously specified data values inorder to generate final decompressed data values for said second dataportion.
 184. The computer system of claim 183, wherein said first dataportion includes one or more position values of a first vertex in saidcompressed 3-D geometry data.
 185. The computer system of claim 184,wherein said second body portion includes one or more vertex parametersof said first vertex selected from the group consisting of: (i) colorvalues, (ii) normal values, (iii) texture map coordinates, and (iv)surface material properties.
 186. A memory media which stores programinstructions for decompressing compressed 3-D geometry data, whereinsaid compressed 3-D geometry data is represented as a data stream whichincludes a plurality of variable-length instructions which describe aplurality of vertices, wherein said plurality of vertex parametervalues, wherein said program instructions are executable to implementthe steps of:receiving said data stream, wherein said data streamincludes a first instruction of said plurality of variable-lengthinstructions, wherein said first instruction includes a fixed-lengthfirst header portion followed in said data stream by a variable-lengthfirst body portion, wherein said first header portion and said firstbody portion include a first data portion having one or more datavalues; accessing said first header portion from said data stream byselecting a predetermined number of bits, wherein said first headerportion includes sufficient information to determine a length value ofsaid first body portion; determining said length value of said firstbody portion from said first header portion; accessing said first bodyportion from said data stream by selecting a number of bits equal tosaid length value of said first body portion; decompressing one or moredata values in said first data portion in accordance with information insaid first header portion.